Neuropilin-1 is a T cell memory checkpoint limiting long-term antitumor immunity.

Robust CD8+ T cell memory is essential for long-term protective immunity but is often compromised in cancer, where T cell exhaustion leads to loss of memory precursors. Immunotherapy via checkpoint blockade may not effectively reverse this defect, potentially underlying disease relapse. Here we report that mice with a CD8+ T cell-restricted neuropilin-1 (NRP1) deletion exhibited substantially enhanced protection from tumor rechallenge and sensitivity to anti-PD1 immunotherapy, despite unchanged primary tumor growth. Mechanistically, NRP1 cell-intrinsically limited the self-renewal of the CD44+PD1+TCF1+TIM3- progenitor exhausted T cells, which was associated with their reduced ability to induce c-Jun/AP-1 expression on T cell receptor restimulation, a mechanism that may contribute to terminal T cell exhaustion at the cost of memory differentiation in wild-type tumor-bearing hosts. These data indicate that blockade of NRP1, a unique 'immune memory checkpoint', may promote the development of long-lived tumor-specific Tmem that are essential for durable antitumor immunity.

Ebi3 Binding to IFN-γ and IL-10 Limits Their Function.

EBV-induced gene 3 (Ebi3) is a ß subunit within the IL-12 cytokine family that canonically binds to α subunits p19, p28, or p35 to form the heterodimeric cytokines IL-39, IL-27, and IL-35, respectively. In the last decade, the binding partners for Ebi3 have continued to expand to include IL-6 and the other IL-12 family ß subunit p40, revealing the possibility that Ebi3 may be able to bind to other cytokines and have distinct functions. We first explored this possibility utilizing an in vivo mouse model of regulatory T cell-restricted deletions of the subunits composing the cytokine IL-35, p35, and Ebi3, and we observed a differential impact on CD8+ T cell inhibitory receptor expression despite comparable reduction in tumor growth. We then screened the ability of Ebi3 to bind to different cytokines with varying structural resemblance to the IL-12 family α subunits. These in vitro screens revealed extracellular binding of Ebi3 to both IFN-γ and IL-10. Ebi3 bound to IFN-γ and IL-10 abrogated signal transduction and downstream functions of both cytokines. Lastly, we validated that extracellular complex formation after mixing native proteins resulted in loss of function. These data suggest that secreted partnerless Ebi3 may bind to cytokines within the extracellular microenvironment and act as a cytokine sink, further expanding the potential immunological impact of Ebi3.

Regulatory T Cells: Barriers of Immune Infiltration Into the Tumor Microenvironment.

Regulatory T cells (Tregs) are key immunosuppressive cells that promote tumor growth by hindering the effector immune response. Tregs utilize multiple suppressive mechanisms to inhibit pro-inflammatory responses within the tumor microenvironment (TME) by inhibition of effector function and immune cell migration, secretion of inhibitory cytokines, metabolic disruption and promotion of metastasis. In turn, Tregs are being targeted in the clinic either alone or in combination with other immunotherapies, in efforts to overcome the immunosuppressive TME and increase anti-tumor effects. However, it is now appreciated that Tregs not only suppress cells intratumorally via direct engagement, but also serve as key interactors in the peritumor, stroma, vasculature and lymphatics to limit anti-tumor immune responses prior to tumor infiltration. We will review the suppressive mechanisms that Tregs utilize to alter immune and non-immune cells outside and within the TME and discuss how these mechanisms collectively allow Tregs to create and promote a physical and biological barrier, resulting in an immune-excluded or limited tumor microenvironment.