Blockade of Common Gamma Chain Cytokine Signaling with REGN7257, an Interleukin 2 Receptor Gamma (IL2RG) Monoclonal Antibody, in Combination with Costimulatory Blockers Delayed Skin Graft Rejection in Mice

There is a current medical need to identify innovative maintenance therapies to prevent allograft rejection in solid organ transplantation. During transplant rejection, recipient immune cells identify a grafted organ as foreign and attack it: CD4⁺ T cells orchestrate transplant rejection by recruiting a range of effector cells responsible for the damage of rejection including CD8⁺ T cells, NK cells, B cells and macrophages. These pathogenic immune cell responses can be driven by cytokines of the common gamma chain (γ_c) cytokine family (IL2, IL4, IL7, IL9, IL15, and IL21). γ_c cytokines signal through their corresponding receptors, expressed primarily on immune cells, that share a common coreceptor, interleukin 2 receptor subunit gamma (IL2RG) that is required for signaling.

To understand the role of γ_c cytokines in driving transplant rejection, we generated REGN7257, a fully human IL2RG monoclonal antibody that inhibits γ_c cytokine-induced signaling, and we tested its ability to suppress allogeneic immune cell responses in a mouse model of skin graft rejection when delivered as a monotherapy and in combination with mycophenolate mofetil (MMF) or T cell costimulatory pathway blockers. In this model, recipient B6/129 (mixed background) Il2rg^hu/hu mice were transplanted with tail skin from BALB/cJ donor mice. Graft survival was analyzed using a log rank Mantel-Cox test to detect statistically significant differences across groups (*=p<0.033; **= p<0.002; ***=p<0.0002; ****=p<0.0001).

In a mouse model of skin graft rejection, γ_c cytokine signaling blockade with REGN7257 given as maintenance immunosuppression effectively improved overall graft survival (Median Survival Time [MST]_IL2RG=13) compared to mice that received a control antibody (MST_CTRL=9 [****vs IL2RG Ab]), and with similar efficacy to that observed with the calcineurin inhibitor tacrolimus (MST_TAC=12.5 [**vs control, n.s. vs IL2RG Ab]). In treatment combination experiments, while monotherapy treatment with the antiproliferative agent MMF was not as efficient as REGN7257 at delaying skin graft rejection (MST_MMF=10 [*vs control] and MST_IL2RG=12 [*vs MMF]), simultaneous blockade of cell proliferation and γ_c cytokine signaling conferred an advantage over monotherapies (MST_MMF+IL2RG=15.5 [****vs control, **vs IL2RG Ab]). Similarly, induction therapy consisting of γ_c cytokine signaling blockade followed by maintenance immunosuppression using the CTLA4-Ig fusion protein abatacept, significantly improved overall skin graft survival (MST_IL2RG+CTLA4=19.5 [****vs control]) compared to REGN7257 induction only (MST_IL2RG=11 [****vs control and combo]) or abatacept maintenance only (MST_CTLA4=10 [*vs control, ****vs combo]). Finally, maintenance immunosuppression combining blocking of γc cytokine signaling together with blocking of both CD28 and CD40L costimulatory signals prolonged skin graft survival better than any combination therapies of only 2 pathway blockers (MST_TRIPLE=26 [**vs control] and MST_DOUBLE=16-21 [n.s.-** vs triple]).

Overall, our study showed that γ_c cytokine blockade could potentially be an alternative to calcineurin inhibitor-based regimens to prevent rejection by specifically targeting immune cells. Our data also highlighted the inherent complexity of effector mechanisms and pathways involved in allograft rejection and showed that prolonged allograft survival requires simultaneous blockade of multiple T cell activating signals, including inhibition of cytokine signaling and costimulatory signals. These data provide evidence of γ_c cytokines as key drivers of allogeneic immune cell responses during acute transplant rejection, offering a potentially novel strategy for the management of transplant patients.