Genentech continues to work on TIGIT, so what the heck is this target? Lets have a look, but first, some context.
T cell constraint is a fundamental attribute of tumor-induced immunosuppression. CTLA4 and PD-1 are central regulators of this process, and antibody blockade of these pathways can restore anti-tumor responses. The state of T cell constraint (non-responsiveness) has been termed anergy in reference to CD4+ T cells and exhaustion in reference to CD8+ T cells. Exhausted CD8+ T cells have a recognizable T cell phenotype characterized by the expression of diverse inhibitory pathways and proteins, including PD-1, TIM-3, LAG-3 and TIGIT. Whether such a phenotype is absolutely selective for exhausted CD8s is a matter of debate, but is a good starting point for a discussion of the need for so many regulatory pathways.
Dual gene-deficient (knock-out) mice and the administration of blocking antibody combinations have shown that the inhibitory receptors can function synergistically to reject tumors in mouse models. The hypothesis that individual co-inhibitory receptors contribute distinct functions to collectively limit T cell responses has recently been tested in human cancer clinical trials, yielding the impressive result that co-blockade of CTLA4 and PD-1 has synergistic and beneficial anti-tumor activity. Such benefit comes with a toxicity cost, as pathological autoimmunity is revealed when the “brakes” come off the immune system.
Why does the T cell arm of immune system require so many different control pathways? This is a reasonable question, which can be answered somewhat glibly with the observation that uncontrolled immunity leads to autoimmune disease and/or chronic inflammation. Still, though, why are multiple breaks required? The working hypothesis is that one pathway (CTLA4) regulates T cell activation by CD28 that normally occurs in the spleen, lymph nodes, Peyer’s patches and other “secondary lymphoid organs” (the thymus, bone marrow and fetal liver are the major primary lymphoid organs). A second pathway (PD-1) is generally thought to regulate “peripheral” T cell activation at the sites of pathogen encounter – in this sense “peripheral” means outside of the lymphoid organs themselves, that is, in the tissues and circulation, or, in the case of cancer immunology, within the tumor. So, simplistically, there is one control pathway (CTLA4) in the house and another (PD-1) in the yard. The recent paper (link 1) describing the release of T cell recognition of tumor antigens upon CTLA4 blockade in melanoma suggests either cross-talk between the compartment (i.e. tumor beds have lymphatic or circulatory drainage to secondary lymphoid organs) or that the role of CTLA4 is more complex than we think.
What about the other control pathways? LAG-3 is a competitive regulator of CD4/MHCII antigen recognition activity and was shown to confer Treg function when transfected into naive CD4+ T cells. The expression of LAG-3 on CD8+ T cells (which are critical for anti-tumor activity) suggests a role in the interaction of CD4+ and CD8+ T cells. LAG-3 is also expressed on tumor cells and may mask tumors from immune recognition. LAG3/PD-1 doubly gene-deficient mice can reject poorly immunogenic tumors that wild-type mice cannot reject. However, the doubly deficient knockout mice also develop pathological and aggressive autoimmunity. These results show that these proteins have distinct roles in regulating immune responses.
TIM-3 has several immune regulatory activities, one of which is to suppress T cell recognition of phosphatidylserine, a molecule expressed on dead and dying cells but also on tumor cells. As with LAG-3 the combination of anti-PD-1 and anti-TIM-3 antibodies had enhanced anti-tumor efficacy in mouse tumor models when compared to either antibody alone.
And now we have TIGIT, an Ig superfamily protein and a member of the PVR/nectin family that includes CD226 (DNAM-1), CD96, CD112 (PVRL2), and CD155 (PVR), among others. The biology of this family of proteins is complex and a little intimidating. Genentech has been prosecuting this pathway for several years, and their new paper (link 2) has perhaps shed additional light on the biology and utility of this target.
One mechanism by which TIGIT modulated immune responses is via the interaction of TIGIT on T cells with CD155 expressed on immature or resting dendritic cells, which blocks maturation signals normally delivered by CD226, that is, TIGIT is a competitive inhibitor of the interaction of CD226 with CD155. The authors note that this system resembles the co-stimulatory/co-inhibitory receptor pair of CD28 and CTLA-4, where CTLA4 is a competitive inhibitor of the interaction of CD28 with B7-1/CD80 and B7-2/CD86. The expression pattern of the receptors is also similar: both TIGIT and CTLA-4 are induced upon cell activation, while the expression of CD226 and CD28 is constitutive.
As alluded to above, and noted explicitly by the Genentech team, the molecular and functional relationships between TIGIT and it’s various ligands/co-receptors are poorly characterized. Furthermore, TIGIT’s role in regulating CD8+ T cell responses and the mechanisms underlying such regulation are not known. Of note, antibodies to TIGIT or PD-L1 alone enhanced CD8+ T cell effector function in tumor-draining lymph nodes, but blockade of both receptors was required to allow activation of CD8+ T cells within the tumor microenvironment, as measured by IFNy production. The authors conclude that TIGIT is a critical and regulator of CD8+ T cell anti-tumor activity. The mechanism of action evoked to explain the role of TIGIT in the tumor setting was addressed using FRET and other analyses. The authors show that TIGIT interacts directly with CD226 to prevent homodimerization, a component of the interaction of CD226 with CD155.
There are a few things to consider here. The animal models were run with very high amounts of anti-TIGIT and anti-PD-L1 antibodies on board (10 mg/kg anti-PD-L1 and 25 mg/kg anti-TIGIT) given 3 times a week. That’s nearly a gram of antibody approximately every 2.5 days. While the anti-PD-L1 antibody used has a mutated Fc domain that cannot mediate direct cell killing by ADCC, the anti-TIGIT antibody used is a wild-type IgG2a isotype antibody and almost certainly mediates direct killing of TIGIT+ cells. While the in vitro FRET assays are suggestive of the proposed mechanism of action, what is actually occurring in vivo is less clear. TIGIT expression on NK cells is also worthy of further exploration.
So I have a doubt. Not that the pathway is important, but that we really have a good sense of how it functions, nor how antagonism of the pathway in patients will impact anti-tumor activity and baseline immune responses. Locally, Drs Vijay Kuchroo and Ana Anderson have done wonderful work on TIGIT biology, and no doubt one or more of the Cambridge immunotherapy companies is working on this target and exploring it’s utility in the tumor setting. Given the expression pattern of TIGIT in tumors – i.e. on PD-1+/TIM3+ “exhausted” T cells – it is certainly worth the effort to find out.
How to select patients who should respond to anti-TIGIT co-therapy (or anti-TIM-3 or anti-LAG-3) is a critical question, best left for another day.
AND HAPPY HOLIDAYS AND PEACE TO ALL