41BB (CD137) is a costimulatory receptor transiently upregulated on T cells following activation. 41BB is activated by its ligand 41BBL (TNFSF9), a TNF receptor superfamily member expressed by activated antigen presenting cells and anti-41BB agonistic antibodies are in clinical trials for cancer immunotherapy. In a recent article in The Journal of Experimental Medicine, Curran et. al demonstrate that 41BB activation of T cells leads to the generation of a novel subset of CD4+ and CD8+ T cells dependant on the master transcription factor Eomesodermin (Eomes).
Activation of 41BB on T cells leads to enhanced T cell survival. Anti-41BB-agonistic antibodies have demonstrated significant anti-tumor activity in mice by enhancing anti-tumor cytotoxic T cell responses. Thus, there are currently several clinical trials underway exploring the efficacy of anti-41BB-agonist antibodies in several types of cancers, including melanoma, renal carcinoma, ovarian cancer, and lymphoma. In a previous study by the same group (Curran et. al, PLoS One, 2011), an observation was made that a unique subset of T cells infiltrated B16 melanoma tumors in mice after anti-41BB-agonistic antibody treatment. These T cells expressed the inhibitory receptor KLRG1, and elicited strong anti-tumor activity. Thus, in the current study, the authors sought to further characterize this T cell subset in mice.
To define the phenotype and functions of tumor-associated KLRG1+ versus KLRG1- T cells types, T cells were isolated from B16 tumors established in mice, following treatment with anti-41BB antibodies plus irradiated Flt3-ligand–expressing B16 cells (FVAX) or FVAX alone. The addition of FVAX further enhanced the tumor-infiltrating frequency of KLRG1+ T cells elicited by anti-41BB antibodies. Gene expression analysis revealed that KLRG1+ CD4+ and CD8+ T cells expressed significantly higher levels of cytoxicity genes: multiple granzymes, perforin, and FasL, than KLRG1- T cells. In vitro cytotoxicity assays with B16 melanoma cell targets demonstrated enhanced killing capacity of KLRG1+ compared with KLRG1- CD4+ and CD8+ T cells.
Superior cytotoxic functions are generally associated with CD4+ TH1 and CD8+ TC1 T cell subsets, dependant on the transcription factor T-bet (TBX21). However, analysis of expression of the known master transcription factors governing different T cell subsets, found that expression of Eomes but not T-bet was elevated in KLRG1+ T cells. Runx3 expression was also slightly elevated in KLRG1+ versus KLRG1- T cells. Furthermore, transgenic mice lacking Eomes expression in CD4+ cells (CD4-CRE/Eomesflox/flox) did not develop tumor-infiltrating KLRG1+ T cells after anti-41BB antibody treatment, demonstrating the necessity of Eomes for development of these cells, even when Eomes expression is only absent in the CD4+ T cell compartment. Thus, these novel subsets of KLRG1+ T cells were termed CD4+ THEO and CD8+ TCEO T cells.
Interestingly, KLRG1+ T cells play a role not only in anti-tumor immunity, but were induced and found at significant levels in spleens and livers from mice infected with Listeria Monocytogenes or LCMV.
As this is a newly described T cell subset, many questions remain. However, most relevant is whether equivalents of these cells exist in humans, and the roles they play in human diseases.
Systemic 4-1BB activation induces a novel T cell phenotype driven by high expression of Eomesodermin. Curran MA, Geiger TL, Montalvo W, Kim M, Reiner SL, Al-Shamkhani A, Sun JC, Allison JP. J Exp Med. 2013 Apr 8;210(4):743-55.
Combination CTLA-4 blockade and 4-1BB activation enhances tumor rejection by increasing T-cell infiltration, proliferation, and cytokine production. Curran MA, Kim M, Montalvo W, Al-Shamkhani A, Allison JP. PLoS One. 2011 Apr 29;6(4):e19499.
Immunotherapy of cancer with 4-1BB. Vinay DS, Kwon BS. Mol Cancer Ther. 2012 May;11(5):1062-70. doi: 10.1158/1535-7163.MCT-11-0677. Epub 2012 Apr 24.
Immune regulation by 4-1BB and 4-1BBL: complexities and challenges. Wang C, Lin GH, McPherson AJ, Watts TH. Immunol Rev. 2009 May;229(1):192-215.