PlexinA4 mediates cytotoxic T cell trafficking and exclusion in cancer

Cytotoxic T cell (CTL) infiltration of the tumor carries the potential to limit cancer progression, but their exclusion by the immunosuppressive tumor microenvironment hampers the efficiency of immunotherapy. Here, we show that expression of the axon guidance molecule PlexinA4 (Plxna4) in CTLs, especially in effector/memory CD8+ T cells, is induced upon T-cell activation, sustained in the circulation, but reduced when entering the tumor bed. Therefore, we deleted Plxna4 and observed that Plxna4-deficient CTLs acquired improved homing capacity to the lymph nodes and to the tumor, as well as increased proliferation, both achieved through enhanced Rac1 activation. Mice with stromal or hematopoietic Plxna4 deletion exhibited enhanced CTL infiltration and impaired tumor growth. In a melanoma model, adoptive transfer of CTLs lacking Plxna4 prolonged survival and improved therapeutic outcome, which was even stronger when combined with anti-PD-1 treatment. PLXNA4 abundance in circulating CTLs was augmented in melanoma patients versus healthy volunteers but decreased after the first cycle of anti-PD-1, alone or in combination with anti-CTLA-4, in those patients showing complete or partial response to the treatment. Altogether, our data suggest that PlexinA4 acts as a "checkpoint", negatively regulating CTL migration and proliferation through cell autonomous mechanisms independent of the interaction with host-derived PlexinA4 ligands semaphorins. These findings pave the way towards PlexinA4-centric immunotherapies and propose PlexinA4 detection in circulating CTLs as a potential way to monitor the response to immune checkpoint blockade in metastatic melanoma patients.