CD28 blockade controls T cell activation to prevent graft-versus-host disease in primates

Controlling graft-versus-host disease (GVHD) remains a major unmet need in stem cell transplantation, and new, targeted therapies are being actively developed. CD28-CD80/86 costimulation blockade represents a promising strategy, but targeting CD80/CD86 with CTLA4-Ig may be associated with undesired blockade of coinhibitory pathways. In contrast, targeted blockade of CD28 exclusively inhibits T cell costimulation and may more potently prevent GVHD. Here, we investigated FR104, an antagonistic CD28-specific pegylated-Fab', in the nonhuman primate (NHP) GVHD model and completed a multiparameter interrogation comparing it with CTLA4-Ig, with and without sirolimus, including clinical, histopathologic, flow cytometric, and transcriptomic analyses. We document that FR104 monoprophylaxis and combined prophylaxis with FR104/sirolimus led to enhanced control of effector T cell proliferation and activation compared with the use of CTLA4-Ig or CTLA4-Ig/sirolimus. Importantly, FR104/sirolimus did not lead to a beneficial impact on Treg reconstitution or homeostasis, consistent with control of conventional T cell activation and IL-2 production needed to support Tregs. While FR104/sirolimus had a salutary effect on GVHD-free survival, overall survival was not improved, due to death in the absence of GVHD in several FR104/sirolimus recipients in the setting of sepsis and a paralyzed INF-γ response. These results therefore suggest that effectively deploying CD28 in the clinic will require close scrutiny of both the benefits and risks of extensively abrogating conventional T cell activation after transplant.

Biological role of T Memory Stem Cells as a Cellular Reservoir for Graft-versus-Host Disease

T memory stem cells (Tscm) are a subset of antigen-experienced, long-lived, self-renewing and multipotent T cells, which reconstitute immunity following hematopoietic cell transplantation (HCT). Here we investigated the biological role of Tscm in acute Graft-versus-Host Disease (GVHD) using a translational non-human primate model.

Tscm cells were tracked longitudinally following autologous or allogeneic HCT under different immunoprophylaxis regimens, resulting either in fulminant Th1-dependent ‘Primary GVHD’, late-onset Th17-driven ‘Breakthrough GVHD’, or GVHD-free immune tolerance.

Following HCT, Tscm acquired an activated phenotype (Ki67+, CD69+, PD-1+ and CCR5+), but produced low amounts of IFNγ, IL-2, IL-17A, TNFα and Granzyme B. Tscm were a minor population in donor grafts but expanded in peripheral blood (with a reciprocal decline of naïve T cells) early after HCT and then contracted (with a concomitant increase of effector T cells) when recipients developed clinical GVHD. The kinetics of Tscm expansion/contraction correlated closely with clinical disease: Tscm expanded slower and contracted later in animals progressing to Breakthrough compared to Primary GVHD, and Tscm numbers remained stable under tolerizing conditions. At necropsy, animals with GVHD had increased numbers of Tscm in both lymphoid organs (blood, lymph nodes, spleen) and non-lymphoid GVHD-target organs (colon, lungs, liver) compared to GVHD-free animals.

Our data demonstrates that Tscm dynamics correlate with GVHD-free survival and suggest that pathogenic T cells infiltrating both lymphoid and non-lymphoid organs transition from the naïve into the effector state via Tscm. Thus Tscm may play a key role as a cellular reservoir for GVHD.