The plasma membrane tetraspan molecule MS4A4A is selectively expressed by macrophage-lineage cells, but its function is unknown. Here we report that MS4A4A was restricted to murine and human mononuclear phagocytes and was induced during monocyte-to-macrophage differentiation in the presence of interleukin 4 or dexamethasone. Human MS4A4A was coexpressed with M2/M2-like molecules in subsets of normal tissue-resident macrophages, infiltrating macrophages from inflamed synovium, and tumor-associated macrophages. MS4A4A interacted and co-localized with the βglucan receptor Dectin-1 in lipid rafts. In response to Dectin-1 ligands, MS4A4A-deficient macrophages showed defective signaling and defective production of effector molecules. In experimental models of tumor progression and metastasis, MS4A4A deficiency in macrophages had no impact on primary tumor growth, but was essential for Dectin-1mediated activation of macrophages and natural killer (NK) cell-mediated metastasis control. Thus, MS4A4A is a tetraspan molecule selectively expressed in macrophages during differentiation and polarization, essential for Dectin-1 dependent activation of NK cell-mediated resistance to metastasis.

Abundant expression of GNLY in the decidual immunocompetent cells and the capacity of decidual CD56(+) cells to spontaneously secrete high quantities of GNLY point to important protective and immunomodulatory role that this molecule could play at the maternal-fetal interface.

Summary B‐cell acute lymphoblastic leukaemia (B‐ALL) reprograms the surrounding bone marrow (BM) stroma to create a leukaemia‐supportive niche. To elucidate the contribution of immune cells to the leukaemic microenvironment, we investigated the involvement of monocyte/macrophage compartments, as well as several recruitment pathways in B‐ALL development. Immunohistochemistry analyses showed that CD68‐expressing macrophages were increased in leukaemic BM biopsies, compared to controls and predominantly expressed the M2‐like markers CD163 and CD206. Furthermore, the "non‐classical" CD14+CD16++ monocyte subset, expressing high CX3CR1 levels, was significantly increased in B‐ALL patients' peripheral blood. CX3CL1 was shown to be significantly upregulated in leukaemic BM plasma, thus providing an altered migratory pathway possibly guiding NC monocyte recruitment into the BM. Additionally, the monocyte/macrophage chemoattractant chemokine ligand 2 (CCL2) strongly increased in leukaemic BM plasma, possibly because of the interaction of leukaemic cells with mesenchymal stromal cells and vascular cells and due to a stimulatory effect of leukaemia‐related inflammatory mediators. C5a, a macrophage chemoattractant and M2‐polarizing factor, further appeared to be upregulated in the leukaemic BM, possibly as an effect of PTX3 decrease, that could unleash complement cascade activation. Overall, deregulated monocyte/macrophage compartments are part of the extensive BM microenvironment remodelling at B‐ALL diagnosis and could represent valuable targets for novel treatments to be coupled with classical chemotherapy.