Category: PBMC Basics

Innate lymphoid cells (ILCs) are subsets of lymphoid cells that do not rearrange their antigen receptors like T cells and B cells but have other features of lymphocytes.  ILCs include the Natural Killer (NK) cell subset, as well as cells that behave similarly to T helper cell subsets by producing similar characteristic cytokines.  Type 2 ILCs (ILC2) resemble T_H2 cells in that they produce IL-5 and IL-13.  RORγt⁺ ILCs aka ILC3s, include subsets that resemble T_H17 and T_H22 cells by producing IL-17 and IL-22, respectively, as well as a subset that produces both cytokines.   Several recent articles have identified another class of ILCs in both humans and mice.  These cells resemble T_H1 cells in that they express T-bet/TBX21 and produce IFN-gamma, and are distinct from conventional NK cells found among peripheral blood mononuclear cells (PBMC).  These newly characterized cellular subsets have been denoted as Type 1 ILCs (ILC1).

In the April 2013 issue of Immunity, Fuchs et. al sought to more fully characterize the ILC subsets present in human mucosal lymphoid tissues.  In human tonsils, a CD3^–CD56⁺ NKp44⁺CD103⁺ subset was identified that expressed T-bet and produced IFN-gamma when stimulated with either PMA/ionomycin, IL-12, or IL-15.  These cells also expressed perforin and granzyme and had cytolytic activity.  Although these cells express CD56⁺and NKp44⁺, which are markers characteristic of NK cells, they appear to be related to but distinct from prototypical CD56^hi NK cells found in PBMC.  For instance, unlike CD56^hi PBMC NK cells, these ILC1 did not exhibit a response to IL-18, as measured by a synergistic production of IFN-gamma when stimulated with IL-12+ IL-18 vs. IL-12 alone.

In a separate study, published in the March 2013 issue of Nature Immunology, Bernink et. al identified a mucosal human ILC1 subset in tonsils that differs from that found by Fuchs et. al, being CD56^–NKp44⁻ as well asCD127⁺ and c-Kit⁻.  Similarly to the cells described by Fuchs et. al, these cells expressed T-bet and produced  IFN-gamma when stimulated with PMA/ionomycin or IL-12.  However, they did not express perforin and granzyme.  Additional characterizations differentiated these cells from NK cells including the lack of the KIR3DL1 and IL-15Rα markers expressed by NK cells.

ILCs have been found to reside in mucosal associated lymphoid tissues include the oral, lung, and gastrointestinal mucosa, and are thought to function in immune responses to pathogens as well as in tissue repair.  ILCs including ILC3s have also been found to participate in inflammatory disease pathogenesis.  Both types of ILC1 cells were shown to be increased in the intestinal mucosa of Crohn’s disease patients, although their exact locations differed.  CD56⁺ NKp44⁺CD103⁺ cells were found to accumulate in the intraepithelial layer while CD127⁺CD56^–c-Kit⁻NKp44⁻ cells were found in the lamina propria.  Thus, these two subsets of ILC1 cells differ in multiple aspects including tissue localization.

In conclusion, both types of ILC1 cells identified in these studies are distinct from conventional PBMC CD56^hi NK cells, express T-bet, and produce IFN-gamma in response to IL-12 and IL-15 stimulation.  Notably, ILC3 cells also heterogeneously express CD56, IFN-gamma, granzymes and perforin.  Thus, many questions remain as to the functional and developmental differences between different ILC subsets and between CD56⁺ ILC1 cells and PBMC NK cells that reside in various tissues.

Reading:

Intraepithelial Type 1 Innate Lymphoid Cells Are a Unique Subset of IL-12- and IL-15-Responsive IFN-γ-Producing Cells.  Fuchs A, Vermi W, Lee JS, Lonardi S, Gilfillan S, Newberry RD, Cella M, Colonna M. Immunity. 2013 Apr 18;38(4):769-81.

Human type 1 innate lymphoid cells accumulate in inflamed mucosal tissues.  Bernink JH, Peters CP, Munneke M, te Velde AA, Meijer SL, Weijer K, Hreggvidsdottir HS, Heinsbroek SE, Legrand N, Buskens CJ, Bemelman WA, Mjösberg JM, Spits H.  Nat Immunol. 2013 Mar;14(3):221-9.

ILC1 Populations Join the Border Patrol.  Maloy KJ, Uhlig HH. Immunity. 2013 Apr 18;38(4):630-2. doi: 10.1016/j.immuni.2013.03.005.

Innate lymphoid cells: emerging insights in development, lineage relationships, and function.  Spits H, Cupedo T. Annu Rev Immunol. 2012;30:647-75.

A T-bet gradient controls the fate and function of CCR6-RORγt+ innate lymphoid cells.  Klose CS, Kiss EA, Schwierzeck V, Ebert K, Hoyler T, d’Hargues Y, Göppert N, Croxford AL, Waisman A, Tanriver Y, Diefenbach A.  Nature. 2013 Feb 14;494(7436):261-5.

Probably the most common way to cryopreserve cells, including human peripheral blood mononuclear cells (PBMC) is using a mixture of 90% serum with 10% DMSO.  However, serum is very expensive, and every new lot must first be tested for its effects on the background and performance of the various cellular assays performed.  A recent article in Cancer, Immunology, Immunotherapy, by Filbert et. al, reports on the results of an effort led by the Cancer Immunotherapy Immunoguiding Program to compare the viability, recovery, and performance in IFN-gamma ELISPOT assays of PBMCs cryopreserved in serum-containing versus various serum-free mediums.

This was a large-scale study which engaged 31 labs across ten countries.  This study is part of a larger concerted effort by the Immunoguiding Program of the Cancer Immunotherapy Association and the Cancer Research Institute’s Cancer Immunotherapy Consortium to assess the importance of harmonizing the most commonly utilized immunological assays across institutions, such that standardized results can be obtained.  The major inertia driving this effort is to establish a platform for standardized evaluation of patient immune responses to support the growing field of clinical immunotherapeutics.

In this study, three different freezing media were compared in 31 labs and seven freezing media were compared in a single center.  Human PBMCs from HLA-A*0201 donors were cryopreserved in these various freezing mediums and sent to the different labs for evaluation of viability, recovery, and performance in IFN-gamma ELISPOT protocols against several HLA-A*0201-restricted epitopes from HCMV, Influenza, and EBV viruses.  Each lab used its own established ELISPOT protocol.

All 31 labs compared PBMCs cryopreserved in (1) 90 % heat-inactivated human AB serum + 10 % DMSO, (2) CryoMaxx II, and (3) 10 % human serum albumin (HSA) + 10 % DMSO + 80 % RPMI.  Interestingly, the viability of cells after thawing as well as the number of cells recovered after thawing and after a 1-24 hour rest, were found to be significantly higher in both serum-free mediums compared to the human AB serum-containing media.  The overall cell loss from the number of cells initially cryopreserved ended up being an average of 35.2 % for PBMCs cryopreserved in the human AB serum-containing media, and roughly 22% for both of the serum-free mediums.  Thus, these assays suggest that these serum-free mediums provide more optimal freezing conditions compared with the human AB serum-containing media.  The performance in ELISPOT assays however, was not found to be significantly different for cells frozen in these different mediums.

In addition to those three mediums, a single laboratory made the same assessments for PBMCs cryopreserved in an additional four mediums: (4) CryoKit ABC, (5) 90 % heat-inactivated FCS + 10 % DMSO, (6) 12.5 % BSA + 77.5 % RPMI + 10 % DMSO, and (7) 12.5 % BSA + 77.5 % RPMI + 5 % DMSO + 5 % hydroxyethyl starch. In this comparison however, serum-free and serum-containing mediums had similar effects on viability, cell recovery, and in the ELISPOT assay, although the BSA-containing mediums had the worst performance overall.

In conclusion, although commonly used FBS and FCS-containing mediums were not compared in the multi-lab test, the strong performance of cells cryopreserved in serum-free media regarding subsequent viability, recovery, and in ELISPOT assays recommends that further consideration be given to cryopreservation in such serum-free media. Long term storage quality of cells frozen in various serum-free media is still an issue to be addressed as well as the comparative performance of PBMCs in the many other immunological assays.  Using defined serum-free media as opposed to lot-variant serum-containing media may allow for more robust standardization of immunological assays.

Serum-free freezing media support high cell quality and excellent ELISPOT assay performance across a wide variety of different assay protocols.  Filbert H, Attig S, Bidmon N, Renard BY, Janetzki S, Sahin U, Welters MJ, Ottensmeier C, van der Burg SH, Gouttefangeas C, Britten CM. Cancer Immunol Immunother. 2013 Apr;62(4):615-27. doi: 10.1007/s00262-012-1359-5. Epub 2012 Nov 9.

The impact of harmonization on ELISPOT assay performance.  Janetzki S, Britten CM. Methods Mol Biol. 2012;792:25-36. doi: 10.1007/978-1-61779-325-7_2.

Harmonization of immune biomarker assays for clinical studies.  van der Burg SH, Kalos M, Gouttefangeas C, Janetzki S, Ottensmeier C, Welters MJ, Romero P, Britten CM, Hoos A. Sci Transl Med. 2011 Nov 9;3(108):108ps44. doi: 10.1126/scitranslmed.3002785.

Standardized Serum-Free Cryomedia Maintain Peripheral Blood Mononuclear Cell Viability, Recovery, and Antigen-Specific T-Cell Response Compared to Fetal Calf Serum-Based Medium.  Germann A, Schulz JC, Kemp-Kamke B, Zimmermann H, von Briesen H. Biopreserv Biobank. 2011 Sep;9(3):229-236.

Dendritic cells (DC) are major antigen-presenting cells consisting of numerous heterogeneous subtypes.   In humans, several subtypes of DCs have been identified in different tissues including peripheral blood, secondary lymphoid organs, and in the skin.  In peripheral blood mononuclear cells (PBMC) and secondary lymphoid organs, these subtypes include BDCA1⁺, BDCA3⁺, and plasmacytoid DCs which differ functionally and in expression of various markers. So how do these many human DC subsets differ functionally?

Antigen cross-presentation is a DC-specialized mechanism by which antigens are taken up through endocytic and phagocytic pathways but presented in the context of MHC-class I, to activate antigen-specific cytotoxic CD8 T cells.  Recent studies have sought to characterize the differences between the many tissue-associated DC subsets including their ability to cross-present antigen.

In PBMC, DCs are quite rare, comprising only 1 – 2% of PBMCs.  In previous blog posts, the generation of dendritic cells from PBMC monocytes and maturing and assaying monocyte-derived dendritic cells have been discussed.  However, blood DCs and in vitro generated DCs may not represent the true physiological state of DCs present in secondary lymphoid organs where natural antigen cross-presentation and T cell activation occur in vivo.

In a recent study in The Journal of Experimental Medicine, Segura et. al explored the antigen cross-presentation versus phagocytic capabilities of human BDCA1⁺, BDCA3⁺, and plasmacytoid DC subsets compared with CD11c⁺HLADR⁺CD14⁺ macrophages that were all freshly isolated from healthy donor tonsils.  The cross-presentation capabilities of different types of antigens, including necrotic dead cell antigens and soluble antigens were assessed.

For necrotic dead cell antigens, such as dead tumor cells, BDCA1⁺ and BDCA3⁺ DC subsets both took up antigens to a similar extent and were the most efficient in activating CD8⁺ T cell responses which were measured by IFN-gamma production in allogeneic mixed leukocyte reaction assays.  Macrophages far exceeded the ability of any DC subsets in dead cell phagocytosis, but were extremely poor at cross-presentation and CD8⁺ T cell activation.  Plasmacytoid DCs were the poorest at phagocytosis of dead cells, and were also unable to cross-present these antigens.  For soluble antigens however, all three DC subsets (BDCA1⁺, BDCA3⁺, and plasmacytoid DC) efficiently cross-presented both shorter and longer soluble peptides, while macrophages continued to be poor at cross-presentation.

In an additional set of assays, the authors explored mechanisms that may contribute to the differential phagocytosis versus antigen cross-presentation of DC subsets and macrophages.    Compared with macrophages which did not cross-present antigens, the endocytic compartments of cross-presenting DCs kept an alkaline pH and contained reactive oxygen species, and these DCs further were able to export internalized antigens to the cytosol where they can be loaded onto MHC-class I.  Thus, while macrophages are efficient phagocytes, they are unable to process antigens to allow for cross-presentation.

In conclusion, understanding the capabilities of immune cells in different tissues is critical to discovering the full spectrum of cellular functions.  DCs are a major target for vaccinations and immunotherapeutic strategies, and describing and understanding these subsets in vivo will lead to maximized success in immune modulating modalities.

Further Reading:

Similar antigen cross-presentation capacity and phagocytic functions in all freshly isolated human lymphoid organ-resident dendritic cells.  Segura E, Durand M, Amigorena S. J Exp Med. 2013 Apr 8.

Cross-presentation by dendritic cells.  Joffre OP, Segura E, Savina A, Amigorena S.  Nat Rev Immunol. 2012 Jul 13;12(8):557-69. doi: 10.1038/nri3254. Review.

BDCA-2, BDCA-3, and BDCA-4: three markers for distinct subsets of dendritic cells in human peripheral blood. Dzionek, A., A. Fuchs, P. Schmidt, S. Cremer, M. Zysk, S. Miltenyi, D.W. Buck, and J. Schmitz. 2000. J. Immunol. 165:6037–6046.

Characterization of resident and migratory dendritic cells in human lymph nodes.  Segura, E., J. Valladeau-Guilemond, M.H. Donnadieu, X. Sastre-Garau, V. Soumelis, and S. Amigorena. 2012. J. Exp. Med. 209:653– 660.

Gene family clustering identifies functionally associated subsets of human in vivo blood and tonsillar dendritic cells. Lindstedt, M., K. Lundberg, and C.A. Borrebaeck. 2005. J. Immunol. 175:4839–4846.

Functional specializations of human epidermal Langerhans cells and CD14+ dermal dendritic cells.  Klechevsky, E., R. Morita, M. Liu, Y. Cao, S. Coquery, L. Thompson- Snipes, F. Briere, D. Chaussabel, G. Zurawski, A.K. Palucka, et al. 2008. Immunity. 29:497–510.

Characterization of dermal dendritic cells obtained from normal human skin reveals phenotypic and functionally distinctive subsets. Nestle, F.O., X.G. Zheng, C.B. Thompson, L.A. Turka, and B.J. Nickoloff. 1993. J. Immunol. 151:6535–6545.

Natural killer (NK) cells represent up to 15% of human peripheral blood mononuclear cells (PBMC), and range from 5-20% of peripheral blood lymphocytes.  NK cells generally fall into three subtypes: CD56^dim CD16⁺, CD56^brightCD16^+/- and CD56^– CD16⁺ NK cells, the prevalence and functions of which I have previously discussed.  NK cells are considered to be a promising avenue in cell-based anti-tumor immunotherapeutics.  However, the relatively low numbers of these cell types in PBMC have constituted a technical challenge in these efforts and in other studies needing large numbers of NK cells.  In the April 2013 issue of Clinical & Experimental Immunology, Wang et. al, describe an in vitro method for the preferential expansion of human NK cells from PBMC.

NK cell expansion in vitro systems requires multiple signals for survival, proliferation, and activation.  In a previous study, Fujisaki et. al (2009) demonstrated that highly cytotoxic CD56⁺ NK cells could be highly preferentially expanded when cultured with a version of the chronic myeloid leukemia K562 cell line, which was genetically altered to express a membrane-bound form of IL-15 and the 41BB ligand (CD137L).  Under this protocol, NK cells expanded an average of 21.6-fold after 7 days and 277-fold after 21 days in culture, and at 21 days reached a purity of 98.6%.  CD3⁺ T cells on the other hand fell to an average of 3.1% of the cells remaining after 21 days.  Importantly however, is not only the expansion of NK cells, but the functionality of the expanded cell product.  The NK cells generated by this method had enhanced killing potential in vitro.  In xenograft models of acute myeloid leukemia (AML) in immune deficient NOD/scid-IL2RGnull mice, these NK cells were able to elicit potent anti-leukemic activity.  Thus, this method generates large numbers of highly functional human NK cells.

In the current study by Wang et. al, a similar method was utilized in which the K562 cell line was engineered to express a membrane-bound form of IL-21 along with CD137L.  On average under these conditions, NK cells expanded from less than 30% of PBMC to over 85% after 7 days and 95% after 3 weeks, while CD3⁺ T cells went from 60% initially to 6% at seven days and 1% at three weeks.  Proliferation of NK cells was continual over eight weeks in culture, and by 3 weeks reached over 100-fold, although the exact numbers and ranges were not explicitly stated in the paper.  Thus, NK cells are highly selectively expanded using this method, similarly to the method used by Fujisaki et. al.

In answer to the functionality of NK cells generated under these conditions, Wang et. al demonstrated enhanced expression of activating and inhibitory NK receptors.  Significantly enhanced cytotoxic killing potential after culture was shown, being maximal after one and three weeks in culture whereafter it decreased but still remained higher than resting NK cells.  Thus, these expanded NK cells are also highly functional.

It would be interesting to see a direct comparison of the extent and quality of NK cell expansion from human PBMC by CD137L combined with the membrane-bound form of IL-15 as was done by Fujisaki et. al versus the membrane-bound form of IL-21 developed by Wang et. al.  IL-21 is a strong and preferential activator of STAT3.  Wang et al did establish a role for STAT3 in the induction of these cells.  IL-15 is a strong activator of STAT5 and activates STAT3 to a lesser extent.  However, IL-15 has been shown to strongly induce expression of the STAT3-activating cytokine IL-10.  Thus, for optimal clinical applications of expanded NK cells, it is important to determine how the different cytokine-STAT signals contribute to NK cell proliferation, survival, and activation.

Further Reading:

Membrane-bound interleukin-21 and CD137 ligand induce functional human natural killer cells from peripheral blood mononuclear cells through STAT-3 activation.  Wang X, Lee DA, Wang Y, Wang L, Yao Y, Lin Z, Cheng J, Zhu S. Clin Exp Immunol. 2013 Apr;172(1):104-12. doi: 10.1111/cei.12034.

Expansion of highly cytotoxic human natural killer cells for cancer cell therapy.  Fujisaki H, Kakuda H, Shimasaki N, Imai C, Ma J, Lockey T, Eldridge P, Leung WH, Campana D. Cancer Res. 2009 May 1;69(9):4010-7. doi: 10.1158/0008-5472.CAN-08-3712. Epub 2009 Apr 21.

Natural Killer Cell subtypes and markers in human PBMC

Types of immune cells present in human PBMC

Prospects for the use of NK cells in immunotherapy of human cancer.  Ljunggren HG, Malmberg KJ. Nat Rev Immunol. 2007 May;7(5):329-39.

Properties of the K562 cell line, derived from a patient with chronic myeloid leukemia.  Klein E, Ben-Bassat H, Neumann H, Ralph P, Zeuthen J, Polliack A, Vánky F. Int J Cancer. 1976 Oct 15;18(4):421-31.

Characterization of cytokine differential induction of STAT complexes in primary human T and NK cells.  Yu CR, Young HA, Ortaldo JR. J Leukoc Biol. 1998 Aug;64(2):245-58.

IL-15-induced IL-10 increases the cytolytic activity of human natural killer cells.  Park JY, Lee SH, Yoon SR, Park YJ, Jung H, Kim TD, Choi I. Mol Cells. 2011 Sep;32(3):265-72. doi: 10.1007/s10059-011-1057-8. Epub 2011 Jul 29.

Immunologists study many aspects regarding differentiation of T cells and function of T cell lineages.  The results and interpretations from these studies always rely on the robustness of the experimental setup.  A question that I posed to myself recently, when testing protocols for differentiation of naïve CD4⁺ T cells into various functional lineages (T_H1, T_H2, T_H17, T_REG), was whether or not CD4⁺ terminally differentiated effector memory (T_EMRA) cells are still present in the population of “naïve” CD4⁺ T cells obtained following isolation from peripheral blood mononuclear cells (PBMC).

Following antigen exposure, CD4⁺ and CD8⁺ T cells undergo differentiation thorough various stages.  While the exact path of differentiation remains under exploration, a current mainstream hypothesis is that naïve cells (T_N) progress through central memory (T_CM), then effector memory (T_EM), then finally terminally differentiated effector memory (T_EMRA) states.  Expression of surface markers have been used to identify human T cells in these various states, including CD45RA, CD45RO, CCR7, CD62L, CD27, and CD28.  After antigen exposure, naïve T cells, which are CD45RA⁺CD45RO^–CCR7⁺CD62L⁺CD27⁺CD28⁺ lose expression of CD45RA and gain expression of CD45RO.  As memory T cells progress from T_CM to T_EM cells, they additionally lose expression of CCR7, CD45RA⁺, CD27, and CD28.  Finally, T_EMRA cells regain expression of CD45RA, but remain identifiable from naïve T cells by their lack of CCR7, CD62L, CD27, and CD28 expression.

The function of CD4⁺ T_EMRA cells parallels that of CD8⁺ T_EMRA cells.  These cells are cytolytic and express IFN-gamma after activation through their TCR or stimulation with PMA/ionomycin. CD4+ T_EMRA cells also have shorter telomeres than naïve, T_CM, and T_EM populations, and lower homeostatic proliferation capacity.

While T_EMRA cells are well described for CD8⁺ T cells, they often are ignored as part of the CD4⁺ compartment.  Despite the lack of attention that  CD4⁺ T_EMRA cells are given in the literature, I observe them quite frequently in human PBMC from healthy donors, on average being 4% of CD4⁺ T cells (range 0-15%) and 11% of CD4⁺CD45RA⁺ cells (range 0-40%).  Additionally, the percentage of CD4⁺ T_EMRA cells that I observe has a strong correlation with IFN-gamma production by CD4⁺CD45RA⁺ T cells from the same donor.

Figure: A. Expression of CD45RA vs. CD62L in human CD4⁺ PBMCs from four donors.  CD4⁺ T_EMRA cells are CD45RA⁺CD62L^– (lower right quadrant). B. Expression of CD45RA vs. IFN-gamma in human CD4⁺ PBMCs from the same four donors stimulated with PMA/ionomycin. CD45RA⁺IFN-gamma⁺ cells are likely CD4⁺ T_EMRA cells.

Considering CD4⁺ T_EMRA cells are not only commonly present but highly functional, I wondered if they would be present in the population of “naïve” CD4⁺ T cells obtained following isolation from PBMC.  If fluorescence-activated cell sorting (FACS) is used for cell isolation, then this is an easier issue to avoid as all of the necessary markers used to differentiate naïve CD4⁺ T cells from other cell subsets can be included in the marker staining panel.  However, many researchers use commercially available magnetic bead-based kits or other similar methodologies to obtain a “naïve” CD4⁺ T cell population.  Because there is no single marker that would isolate a naïve CD4 T cell from PBMC, negative selection kits for untouched isolation of naïve CD4⁺ T cells are commercially available as “one-step” kits.  These are available from companies including Miltenyi Biotec, Stem Cell Technologies, and R&D Systems.

Analysis of the antigens negatively selected for by these kits revealed the following lists: Miltenyi Biotec: CD45RO, CD8, CD14, CD15, CD16, CD19, CD25, CD34, CD36, CD56, CD123, TCRγ/δ, HLA-DR, and CD235a (glycophorin A).

Stem Cell Technologies: CD45RO, CD8, CD14, CD16, CD19, CD20, CD36, CD56, CD66b, CD123, TCRγ/δ, and CD235a (glycophorin A).

Unfortunately, nothing in the literature indicated that any of these markers targeted removal of T_EMRA cells, and the manufacturer’s data sheets only show that the final product obtained by using these kits are CD4⁺CD45RA⁺ cells.  Technical support from both Miltenyi Biotec and Stem Cell Technologies came to the same conclusion: the CD4⁺ T_EMRA cells are not removed.

The conclusion:  Isolation of naïve CD4 cells without T_EMRA cells may still be possible if this is necessary for your assays.  Following usage of one of the above mentioned kits, positive selection for CCR7, CD62L, CD27, or CD28 can be tested.

The final question is whether these cells can affect the experimental results from for instance, studies on T-helper (T_H) subset differentiation.  While it is unknown if these cells themselves could differentiate into T_H subtypes, they certainly can produce IFN-gamma in culture which inhibits T_H subset differentiation along non-T_H1 lineages, underscoring the necessity for the inclusion of anti-IFN-gamma antibodies when differentiating T_H subtypes other than T_H1.

Further reading:

eBiosciences Human CD & Other Cellular Antigens Chart

**Phenotypic heterogeneity of antigen-specific CD4 T cells under different conditions of antigen persistence and antigen load.  Harari A, Vallelian F, Pantaleo G. Eur J Immunol. 2004 Dec;34(12):3525-33.

Phenotype and function of human T lymphocyte subsets: consensus and issues.  Appay V, van Lier RA, Sallusto F, Roederer M. Cytometry A. 2008 Nov;73(11):975-83. doi: 10.1002/cyto.a.20643.

Phenotypic and functional profiling of CD4 T cell compartment in distinct populations of healthy adults with different antigenic exposure.  Roetynck S, Olotu A, Simam J, Marsh K, Stockinger B, Urban B, Langhorne J. PLoS One. 2013;8(1):e55195. doi: 10.1371/journal.pone.0055195. Epub 2013 Jan 28.

Sensitive gene expression profiling of human T cell subsets reveals parallel post-thymic differentiation for CD4+ and CD8+ lineages. Appay V, Bosio A, Lokan S, Wiencek Y, Biervert C, Küsters D, Devevre E, Speiser D, Romero P, Rufer N, Leyvraz S. J Immunol. 2007 Dec 1;179(11):7406-14.

Characterization of CD4(+) CTLs ex vivo.  Appay V, Zaunders JJ, Papagno L, Sutton J, Jaramillo A, Waters A, Easterbrook P, Grey P, Smith D, McMichael AJ, Cooper DA, Rowland-Jones SL, Kelleher AD. J Immunol. 2002 Jun 1;168(11):5954-8.

Altered proportions of naïve, central memory and terminally differentiated central memory subsets among CD4+ and CD8 + T cells expressing CD26 in patients with type 1 diabetes.  Matteucci E, Ghimenti M, Di Beo S, Giampietro O. J Clin Immunol. 2011 Dec;31(6):977-84. doi: 10.1007/s10875-011-9573-z. Epub 2011 Sep 2.

Negative regulatory CD4 T cells are well characterized and highly studied.  However their CD8 counterparts are not well defined, particularly in humans.  Regulatory CD8 T cells suppress activated CD4 T cells and have proposed roles in various human diseases including multiple sclerosis, ovarian carcinoma and infection with HIV, and many subsets have been described using various markers.  In a recent issue of PLoS One, Hu et. al, describe a population of CD3⁺CD8⁺CD161⁻CD56⁺ T cells within human peripheral blood mononuclear cells (PBMC) that exhibit a cytolytic negative regulatory function.

This group previously published a study where they isolated CD8 T cell clones that were able to lyse autologous T cell receptor (TCR) activated CD4 T cells (Hu et al., 2011).  Surface marker characterization of these regulatory CD8 T cell clones by flow cytometry found that they expressed CD56, CD62L and CD95 but not CD16, CD161, CXCR4 and CCR7.

Because CD161 and CD56 are generally co-expressed markers in NK and NKT cells but are not expressed on conventional CD8 T cells, the authors reasoned that these markers (CD161⁻CD56⁺) in addition to CD3 and CD8 may provide a robust way to distinguish this population of regulatory CD8 T cells from conventional CD8 T cells, NK cells, and NKT cells by flow cytometry.  Thus in the PLoS One study, the author’s objectives included identification and characterization of this subset of regulatory CD8 T cells in normal human PBMC.

A population of CD3⁺CD8⁺CD161⁻CD56⁺ regulatory CD8 T cells were identified in PBMC and compared with conventional CD8 T cells (CD3⁺CD8⁺CD161⁻CD56^–) and NKT cells (CD3⁺CD8⁺CD161⁺CD56⁺).  On average, regulatory CD8 T cells occurred at a frequency of 3.2% of total CD8 T cells.  Regulatory CD8 T cells resembled terminally differentiated effector CD8 cells by expressing CD45RA, but not CD45RO or CCR7, and had lower levels of CD62L and CD27.  NKT cells in contrast expressed CD45RO.  For a further discussion of expression of CD45RA, CD45RO, CCR7, CD62L, and CD27 by naïve, central memory, effector memory, and terminally differentiated effector T cell populations, I refer you to a previous post.

Expression of these and numerous other markers were examined in resting and activated regulatory CD8 T cells including CD127, CD25, CD28, CD69, CD94, NKG2a, CD8β, and TCRVα24, and the details can be found in the paper.  Additionally, morphological examination of these cells revealed a larger cytoplasm with some granules, and an irregular nucleus, characteristic of activated T cells and NK cells, but not resting conventional CD8 T cells.

Finally the authors demonstrated that activated CD56⁺ but notCD56^–, CD8⁺CD161^– T cells could lyse autologous and allogeneic activated CD4 T cell targets, similarly to the regulatory CD8 T cell clones previously described.

Thus, this study describes the identification of a CD161⁻CD56⁺ CD8 T cell subset capable of negative regulatory function: cytolysis of activated CD4 T cells.  Many questions remain for further exploration of this interesting population of cells.  Multiple other negative regulatory CD8 T cell subsets have been described including FoxP3+ CD8 T cells.  Determining the differences between various regulatory CD8 T cell subsets regarding marker expression and function should be addressed.  Additionally, the CD8 T cell clones previously described by this group expressed IFN-gamma following activation.  As these negative regulatory CD8 T cells also phenotypically resemble terminally differentiated effector CD8 cells, these populations should be directly functionally compared in future studies.

Identification of Cytolytic CD161(-)CD56(+) Regulatory CD8 T Cells in Human Peripheral Blood.  Hu D, Weiner HL, Ritz J. PLoS One. 2013;8(3):e59545. doi: 10.1371/journal.pone.0059545. Epub 2013 Mar 19.

A clonal model for human CD8+ regulatory T cells: unrestricted contact-dependent killing of activated CD4+ T cells.  Hu D, Liu X, Zeng W, Weiner HL, Ritz J.  Eur J Immunol. 2012 Jan;42(1):69-79. doi: 10.1002/eji.201141618. Epub 2011 Nov 28.

Basic markers of T cell populations in human PBMC

Flow cytometry has been around since the 1950s when Wallace Coulter developed the first flow cytometry device and fluorescence-based flow cytometry was introduced in 1968 by Wolfgang Göhde.  Since then, fluorescence-based flow cytometry and fluorescence-activated cell sorting (FACS) have blown up to become a mainstay of analytical scientific approaches in every field of cell biology, especially immunology.  However, the dominance of fluorescence-based flow cytometry for analytical cellular biology may change with the recent introduction of a new technology: Time of Flight Mass Cytometry (CyTOF).

In fluorescence-based flow cytometry, cells or particles labeled with fluorescent dye-conjugated antibodies or other fluorescent proteins flow in a single file stream past a series of lasers that emit light at specific wavelengths, causing the fluorescent dyes to become excited and emit light caught by detectors.  Thus, a quantitative measure of intensity for each fluorescent parameter, pertaining to the expression level of the antibody-targeted antigen of interest, is obtained for every cell.  The BD Biosciences Influx cell sorter is currently a top of the line fluorescence-based flow cytometer, and supports up to 10 lasers and detection of up to 24 parameters.  However, even with a thorough understanding of flow cytometry, the actual number of utilizable parameters will be typically be far less due to limitations including spectral overlap of fluorescent dyes.

CyTOF utilizes an entirely different technique to quantify protein expression levels on a single cell level: the use of transition element isotopes to label antibodies.  The quantities of isotopes bound to each cell are then analyzed by a time-of-flight mass spectrometer.  While compensation issues due to spectral overlap between fluorophores limits the effective number of parameters assessable by fluorescence-based flow cytometry to far below the theoretical maximums, CyTOF does not suffer from these limitations as there is no requirement for compensation.  In addition, as the metal isotopes used are rare, there is no autofluorescence of cells, another limitation of fluorescence-based flow cytometry. Proof of principle studies have been published by Gary Nolan and colleagues at Stanford University, and have demonstrated the simultaneous use of 34 cell surface and intracellular parameters.  The CyTOF instrument can theoretically detect up to 100 isotopes, thus far extending the ability of researchers to simultaneously assess the expression of many more proteins per cell.

The CyTOF instrument is commercially available from DVS Sciences.  DVS Sciences also offers an expanding list of pre-conjugated metal isotope-labeled antibody reagents and additionally a MAXPAR® labeling kit for conjugation of other antibodies to 33 different metals, allowing researchers to select many additional antigens of interest for analysis.

I have previously stressed the importance of studying cell biology on the single cell level in order to understand the relationships that occur between expression of proteins and signaling states in unique cell populations and on the single cell level.  The addition of CyTOF to the reseracher’s arsenal will allow these types of questions to be addressed on an even more complex level.

 

Additional Reading:

The history and future of the fluorescence activated cell sorter and flow cytometry: a view from Stanford.  Herzenberg LA, Parks D, Sahaf B, Perez O, Roederer M, Herzenberg LA. Clin Chem. 2002 Oct;48(10):1819-27.

Single-cell mass cytometry of differential immune and drug responses across a human hematopoietic continuum.  Bendall SC, Simonds EF, Qiu P, Amir el-AD, Krutzik PO, Finck R, Bruggner RV, Melamed R, Trejo A, Ornatsky OI, Balderas RS, Plevritis SK, Sachs K, Pe’er D, Tanner SD, Nolan GP. Science. 2011 May 6;332(6030):687-96. doi: 10.1126/science.1198704.

Mass cytometry: technique for real time single cell multitarget immunoassay based on inductively coupled plasma time-of-flight mass spectrometry.  Bandura DR, Baranov VI, Ornatsky OI, Antonov A, Kinach R, Lou X, Pavlov S, Vorobiev S, Dick JE, Tanner SD. Anal Chem. 2009 Aug 15;81(16):6813-22. doi: 10.1021/ac901049w.

Cytometry by time-of-flight shows combinatorial cytokine expression and virus-specific cell niches within a continuum of CD8+ T cell phenotypes. Immunity. 2012 Jan 27;36(1):142-52. doi: 10.1016/j.immuni.2012.01.002.

Tricks for analyzing PBMC populations by flow cytometry

photo credit: PNNL – Pacific Northwest National Laboratory via photopincc

The availability of human peripheral blood mononuclear cells (PBMC) from healthy individuals and from patients with various diseases allows for many studies on normal and abnormal functions of human immune cells.  Because human and murine immune biology differs in many ways, it is important that various methodologies for studying human immunology are established.  The two reports highlighted below demonstrate the usage of human PBMCs for mechanistic and pre-clinical human immune cell studies.

A recent report in The Journal of Immunology by Edwards et. al, demonstrated the usage of healthy human PBMC to elucidate the mechanisms involved in modulation of T_H2 T cell responses by Toll-Like Receptor-7 (TLR7) agonists.  Because TLR7 stimulation perturbs T_H2 responses, the potential use of rapidly metabolized 8-oxoadenine TLR7 antedrugs for treatment of allergic diseases was explored in this study in collaboration between AstraZeneca and Dainippon Sumitomo.  The TLR7 agonistic antedrug AZ12441970 was found to inhibit T_H2 responses via at least two different mechanisms: TLR7-induced production of type I interferons (IFN) and induction of Notch-ligand expression on TLR7-responsive antigen presenting cells.  These led to inhibition of IL-5 production by T cells via the IFN and Notch signaling pathways, respectively.  TLR7-induction of IFNα was found to be intact in PBMCs from asthmatic patients when compared with healthy volunteers, and thus the authors proposed that this therapeutic strategy may be effective in allergic disease patients.

This study provides a demonstration of the usage of human PBMCs for elucidating signaling and immune-cell crosstalk mechanisms as well as determining the potential for the effectiveness of candidate drugs in patients with different disease states.

TLR7 Stimulation of APCs Results in Inhibition of IL-5 through Type I IFN and Notch Signaling Pathways in Human Peripheral Blood Mononuclear Cells. Edwards S, Jones C, Leishman AJ, Young BW, Matsui H, Tomizawa H, Murray CM, Biffen M. J Immunol. 2013 Mar 15;190(6):2585-92. doi: 10.4049/jimmunol.1200780.

The role of the cytokine IL-17 in cancer immunity continues to be controversial.  IL-17 and the IL-17-producing T_H17 T cell subsets have been shown to have both pro-tumor as well as anti-tumor immune modulating functions in different cancer contexts.  Monocytes isolated from human PBMC can be differentiated into various myeloid cell types including dendritic cells (DC), providing a tool for studies on these human immune cell types.  In a recent Plos One article, Olsson Åkefeldt et. al, explore the role of IL-17 in survival of human monocyte-derived DCs in vitro, and the relevance of this during chemotherapy.

IL-17 was found to significantly prolong DC survival in vitro, however the cells took on expression of macrophage markers (CD14/CD68) and exhibited a pre-M2 macrophage phenotype.  The prolonged survival was associated with upregulated expression of pro-survival gene BCL-2A1.  Interestingly, IL-17 plus IFNγ treatment in vitro rendered these M2 macrophage-like DCs resistant to cell death induced by 11 of 17 tested chemotherapeutic agents.  Thus, to determine if IL-17 treatment would benefit patients by allowing DC survival during therapy, future studies should address whether this chemoresistance of IL-17 treated DCs occurs in patients undergoing chemotherapy, and to determine how IL-17 affects the anti- versus pro-tumor function of these DCs in various types of cancer.

Chemoresistance of Human Monocyte-Derived Dendritic Cells Is Regulated by IL-17A. Olsson Åkefeldt S, Maisse C, Belot A, Mazzorana M, Salvatore G, Bissay N, Jurdic P, Aricò M, Rabourdin-Combe C, Henter JI, Delprat C. PLoS One. 2013;8(2):e56865. doi: 10.1371/journal.pone.0056865. Epub 2013 Feb 18.

Studies like these are examples of the utility of using human PBMCs to elucidate mechanisms of human immune cell biology under normal and diseased conditions.