Studying the Antagonistic Relationship Between NKG2D and TGF-β Signaling in CD8+ T Cells

Despite the remarkable success of cancer immunotherapies in treating patients with hematological cancers, the clinical response rates of immunotherapies in the case of solid tumors have been dismal. One major reason for the lower response rates is tumor-induced immune-suppression mediated by TGF-?, which directly suppresses the functionality of CD8+ T cells by inhibiting their activation, proliferation, and acquisition of inflammatory cytokines, granzyme B and perforin. On the other hand, NKG2D, a potent co-stimulatory receptor, has effects on CD8+ T cells opposite to that of TGF-?. In T cells from 22 healthy human donors, we found that the suppression by TGF-? was lower in most of the donors with higher expression levels of NKG2D. Based on this and their previously described opposing functional effects, we hypothesized that NKG2D signaling antagonizes the inhibitory effects of TGF-? on CD8+ T cells and makes them more resistant to TGF-? mediated immune-suppression. We show that NKG2D KO CD8+ T cells are indeed far more susceptible to TGF-? mediated inhibition in the production of poly-cytokines and granzyme B, confirming that NKG2D directly opposses TGF-? suppression in CD8+ T cells. Interestingly, stimulatory cytokines IL-2 and IL-15 could not rescue the higher susceptibility to TGF-? in NKG2D KO cells. Further, we investigated the interplay of important co-stimulatory and inhibitory factors with TGF-? and NKG2D signaling. Oppossite to our expectations, we discovered that PD-1 check-point receptor blockade failed to confer resistance to TGF-? in WT CD8+ T cells. On the other hand, CD28, a co-stimluatory receptor earlier shown to overcome TGF-? inhibition, induced TGF-? resistance in NKG2D dependent manner. Finally, using RNA sequencing, we found that the absence of NKG2D signaling in CD8+ T cells led to up-regulation of 22 genes (e.g. IRF7, S100A4, Ahnak) that positively regulate TGF-? and down-regulation of 5 genes (e.g. CD80, PTPN14, NDRG2) that negatively regulate TGF-? pathway. Thus, these findings suggest that NKG2D drives TGF-? resistance mainly by repressing the genes that augment TGF-? signaling. In summary, this dissertation research discovered previously unknown function of NKG2D, as well as the dependence of CD28 on NKG2D, in antagonizing TGF-?’s effects in CD8+ T cells