As is the case in chronic viral infections, both CD4+
tumor infiltrating lymphocytes (TILs) in mice bearing the solid tumors, CT-26, 4T1 and B16, (murine models for colon adenocarcinoma, mammary adenocarcinoma, and melanoma, respectively) co-express Tim-3 and PD-1. Tim-3+
T cells represent a major population within the CD8+
TILs in all three models and virtually all Tim-3+
TILs co-express PD-123
. Interestingly, upregulation of Tim-3 has also been observed in PD-1+
tumor antigen specific CD8+
T cells in the blood of patients with advanced melanoma 22
. In both cases, Tim-3+
cells represent the most impaired population of CD8+
T cells in tumor-bearing hosts in that they exhibit defects in proliferation and produce the least amount of IL-2, TNF, and IFN-γ. Of note, examination of CD8+
TILs in the murine CT-26 tumor model revealed that the PD-1+
single positive TILs that do not express Tim-3 on the cell surface, produced the most IFN-γ among all the populations of TILs. Furthermore, this population was less impaired in progression through cell cycle, as well as in the production of IL-2 and TNF, than the Tim-3+
TILs. Collectively these data support the idea that the Tim-3+
TILs represent the most exhausted TILs and that Tim-3−
TILs may contain a mixture of “exhausted” T cells and bona fide effector T cells.
As the blockade of either the PD-1 or Tim-3 signaling pathways can improve T cell function in the context of chronic infections 20,21,29,53
this raised the possibility that combined targeting of these two pathways may prove to be the most efficacious means to restore anti-tumor immunity in vivo
. Indeed, treatment with anti-Tim-3 alone had little or no effect and treatment with anti-PD-L1 alone showed a trend towards delayed tumor growth that did not reach statistical significance. However, combined treatment with anti-Tim-3 and anti-PD-L1 resulted in a dramatic reduction in tumor growth with 50% of the mice exhibiting complete tumor regression 23
. These data support the notion that the combined targeting of the Tim-3 and PD-1 signaling pathways is effective in restoring anti-tumor immunity in vivo
. Indeed, treatment with anti-Tim-3 plus anti-PD-L1 antibody restores effector function in TILs cultured ex vivo
In line with these observations, the synergistic effect of blocking both pathways was also observed in samples from melanoma patients 22
. When tumor-antigen specific CD8+
T cells from PBMC of melanoma patients were activated in vitro
with specific antigen, the strongest increase in IFN-γ, TNF, and IL-2 production was observed in the cultures when both the Tim-3 and PD-1 signaling pathways were blocked with anti-Tim-3 and anti-PD-L1 antibodies. Interestingly, combination of anti-Tim3 and anti-PD-L1 also increased the frequency of proliferating antigen specific CD8+
cells, thereby increasing the total number of tumor-antigen specific CD8+
T cells in the culture.
T cells that co-express Tim-3 and PD-1 have also been found in mice with advanced acute myelogenous leukemia (AML)24
. Interestingly in AML, the frequency of Tim-3+
T cells also increases as disease progresses. Mirroring the result obtained in solid tumors, the treatment of mice with AML with anti-PD-L1 and Tim-3Ig results in significant reduction in tumor burden and superior survival advantage over mice receiving either treatment alone. Collectively, current data support that the combined targeting of the Tim-3 and PD-1 pathways may be most effective in restoring function of antigen specific CD8+
T cells in both solid and non-solid cancers in humans and as well as in experimental models.
While current data strongly implicate Tim-3 in T cell exhaustion, whether gal-9 or another Tim-3 ligand is involved in T cell exhaustion remains to be addressed. Gal-9 is upregulated by IFNγ 2
; however, T cell exhaustion is associated with low IFNγ, thereby raising the possibility that when gal-9 is low Tim-3+
T cells may escape gal-9 triggered cell death. Counter to this is the observation that in chronic HCV infection, serum levels of gal-9 are high relative to healthy controls54
Clearly, more investigation into the expression of gal-9 in different chronic conditions is required in order to determine its role in T cell exhaustion. (see Box 1
Box1 Tim-3: unanswered questions
T cell exhaustion and cancer
- What factors regulate Tim-3 expression?
- How is Tim-3 taking part in inducing or maintaining exhaustion?
- Is Gal-9 triggering of Tim-3 important in exhaustion?
- Tim-3 is also highly expressed on CD4+ TILs. What is the role of Tim-3 on these cells?
- Why do Tim-3+ cells persist in chronic conditions? Do they escape death or are they constantly being generated?
- How does the Tim-3Gal9 interaction lead to macrophage activation?
- How does Gal-9, a soluble molecule, transduce a signal in innate immune cells to promote IL-1β secretion?
- What are the cell surface interacting partners of Gal9?
- Does Tim-3-mediated T cell exhaustion take place in chronic bacterial infection?
- Do genetic polymorphisms in loci encoding Tim-3 and Gal-9 modulate susceptibility to infectious diseases in humans?