The inflammatory and immune response elicited by the growth of cancer cells is a major element conditioning the tumor microenvironment, impinging on disease progression and patients’ prognosis. Semaphorin receptors are widely expressed in inflammatory cells, and their ligands are provided by tumor cells, featuring an intense signaling cross-talk at local and systemic levels. Moreover, diverse semaphorins control both cells of the innate and the antigen-specific immunity. Notably, semaphorin signals acting as inhibitors of anti-cancer immune response are often dysregulated in human tumors, and may represent potential therapeutic targets. In this mini-review, we provide a survey of the best known semaphorin regulators of inflammatory and immune cells, and discuss their functional impact in the tumor microenvironment.

As a receptor, semaphorin 4C hijacks TGF-β signaling to promote metastatic breast cancer growth.

Semaphorin 4D (Sema4D) plays a role in various cell types including B lymphocytes, differentiating neurons, endothelial cells, and cancer cells. Preclinical and in vitro studies have shown that Sema4D-directed antibodies in combination with immune checkpoint inhibitors reshape the tumor microenvironment by promoting recruitment of effector lymphocytes and antigen-presenting cells, while reducing immunosuppressive cell types, which ultimately leads to tumor rejection. Hence, early-stage clinical trials with combination therapies including anti-Sema4D antibodies are ongoing. In this issue of Cancer Research, Zuazo-Gaztelu and colleagues report an unexpected proinvasive effect induced by anti-Sema4D antibodies in a preclinical model of neuroendocrine pancreatic cancer (Rip1-Tag2), mediated by retrograde signaling of transmembrane Sema4D in macrophages, which increases their recruitment to tumors, SDF-1 secretion, and metastasis-promoting phenotype. See related article by Zuazo-Gaztelu et al., p. 5328

Semaphorin–plexin signaling plays a major role in the tumor microenvironment (TME). In particular, Semaphorin 4D (SEMA4D) has been shown to promote tumor growth and metastasis; however, the role of its high-affinity receptor Plexin-B1 (PLXNB1), which is expressed in the TME, is poorly understood. In this study, we directly targeted PLXNB1 in the TME of triple-negative murine breast carcinoma to elucidate its relevance in cancer progression. We found that primary tumor growth and metastatic dissemination were strongly reduced in PLXNB1-deficient mice, which showed longer survival. PLXNB1 loss in the TME induced a switch in the polarization of tumor-associated macrophages (TAM) toward a pro-inflammatory M1 phenotype and enhanced the infiltration of CD8+ T lymphocytes both in primary tumors and in distant metastases. Moreover, PLXNB1 deficiency promoted a shift in the Th1/Th2 balance of the T-cell population and an antitumor gene signature, with the upregulation of Icos, Perforin-1, Stat3, and Ccl5 in tumor-infiltrating lymphocytes (TILs). We thus tested the translational relevance of TME reprogramming driven by PLXNB1 inactivation for responsiveness to immunotherapy. Indeed, in the absence of PLXNB1, the efficacy of anti-PD-1 blockade was strongly enhanced, efficiently reducing tumor growth and distant metastasis. Consistent with this, pharmacological PLXNB1 blockade by systemic treatment with a specific inhibitor significantly hampered breast cancer growth and enhanced the antitumor activity of the anti-PD-1 treatment in a preclinical model. Altogether, these data indicate that PLXNB1 signaling controls the antitumor immune response in the TME and highlight this receptor as a promising immune therapeutic target for metastatic breast cancers.

The genetic changes sustaining the development of cancers of unknown primary (CUP) remain elusive. The whole‐exome genomic profiling of 14 rigorously selected CUP samples did not reveal specific recurring mutation in known driver genes. However, by comparing the mutational landscape of CUPs with that of most other human tumor types, it emerged a consistent enrichment of changes in genes belonging to the axon guidance KEGG pathway. In particular, G842C mutation of PlexinB2 (PlxnB2) was predicted to be activating. Indeed, knocking down the mutated, but not the wild‐type, PlxnB2 in CUP stem cells resulted in the impairment of self‐renewal and proliferation in culture, as well as tumorigenic capacity in mice. Conversely, the genetic transfer of G842C‐PlxnB2 was sufficient to promote CUP stem cell proliferation and tumorigenesis in mice. Notably, G842C‐PlxnB2 expression in CUP cells was associated with basal EGFR phosphorylation, and EGFR blockade impaired the viability of CUP cells reliant on the mutated receptor. Moreover, the mutated PlxnB2 elicited CUP cell invasiveness, blocked by EGFR inhibitor treatment. In sum, we found that a novel activating mutation of the axon guidance gene PLXNB2 sustains proliferative autonomy and confers invasive properties to stem cells isolated from cancers of unknown primary, in EGFR‐dependent manner.