A New Subset of Negative Regulatory CD8 T Cells in Human PBMC

T cellsNegative 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+CD161CD56+ 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 (CD161CD56+) 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+CD161CD56+ regulatory CD8 T cells were identified in PBMC and compared with conventional CD8 T cells (CD3+CD8+CD161CD56-) 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 CD161CD56+ 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 cellsHu 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

About Andrea

Andrea Miyahira is currently a post-doctoral fellow at the Beckman Research Institute of the City of Hope, in Duarte, CA. She received her Ph.D. from UCLA and her main research interest is in the field of cancer immunology.