Plexin-A4 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 Plexin-A4 (Plxna4) in CTLs, espe-cially in effector/memory CD8(+) T cells, is induced upon T-cell activation, sustained in the circulation, but reduced when enter-ing 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-programmed cell death protein 1 (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-cytotoxic T-Lymphocyte Associated Protein 4 (CTLA-4), in those patients showing complete or partial response to the treatment. Alto-gether, our data suggest that Plxna4 acts as a checkpoint, negatively regulating CTL migration and proliferation through cell-autonomous mechanisms independent of the interaction with host-derived Plxna4 ligands, semaphorins. These findings pave the way toward Plxna4-centric immunotherapies and propose Plxna4 detection in circulating CTLs as a potential way to monitor the response to immune checkpoint blockade in patients with metastatic melanoma.

Automated summary

The infiltration of cytotoxic T cells (CTL) into tumors has the potential to limit cancer progression. However, the immunosuppressive tumor microenvironment hampers the effectiveness of immunotherapy. This study shows that the axon guidance molecule Plexin-A4 (Plxna4) negatively regulates CTL migration and proliferation through cell-autonomous mechanisms. Deletion of Plxna4 improves the homing capacity of CTLs to lymph nodes and tumors, as well as enhances their proliferation. Furthermore, adoptive transfer of Plxna4-deficient CTLs prolongs survival in a melanoma model and shows improved therapeutic outcome when combined with anti-programmed cell death protein 1 (PD-1) treatment. These findings suggest that Plxna4 can serve as a potential target for immunotherapies and its detection in circulating CTLs may be a way to monitor the response to immune checkpoint blockade in melanoma patients with metastasis.