1), in which S1PR5 plays a role in BM egress [16]. To investigate the function of S1PR5 in monocytes, we first compared the percentage of
monocyte subsets in the blood of wild-type (WT) and S1pr5−/− mice [18] by flow cytometry. Results in Figure 2A–C showed a significant reduction of Ly6C− monocytes in the blood of S1pr5−/− mice. This reduction was observed both in S1pr5−/− female (Fig. 2A and B) and male mice (Fig. 2C). S1pr5+/− heterozygous mice also showed a mild phenotype (Fig. 2B). A strong reduction in the frequency 5-Fluoracil cost of Ly6C− monocytes was also observed in the spleen, which is known to be an important reservoir for this subset [19] (Fig. 2D), in the lymph nodes and in non-lymphoid organs such as the lung, liver, and kidney (Fig. 2E). By contrast, the percentage of Ly6C− monocytes appeared normal in the BM of S1pr5−/− mice (Fig. 2F). Moreover, the percentage of Ly6C+ monocytes was normal in
all lymphoid organs of S1pr5−/− mice tested (Fig. 2, all panels). To test if the role of S1PR5 in monocytes was cell-intrinsic, we generated mixed BM chimeras by reconstituting lethally irradiated mice with equal amounts of BM from WT (CD45.1+) and S1pr5−/− (CD45.2+) mice. Six weeks after reconstitution, we measured CD45.1 and CD45.2 expression in different immune subsets in the blood and BM, and calculated the corresponding S1pr5−/− to WT ratio for each subset. As previously reported [20], for Opaganib mw mature NK (mNK) cells, this ratio was very high in the BM and very low in the blood (Fig. 3, left panel), reflecting the important role of S1PR5 in NK cell exit from the BM. For Ly6C+ monocytes, the S1pr5−/− to WT ratio was
nearly 1 in both blood and BM (Fig. 3, right panel), confirming the absence of a role of S1PR5 in this subset. By contrast, for Ly6C− monocytes, the S1pr5−/− to WT ratio was near 0.5 in the BM and 0.1 in the blood (Fig. 3, left panel). These data suggest that S1PR5 is important both for the development of Ly6C− monocytes and for their trafficking or their survival DCLK1 at the periphery. The paucity of patrolling monocytes in the periphery of S1pr5−/− mice could be explained by a role of this receptor either in their egress from the BM or in their survival at the periphery. To try and discriminate between both hypotheses, we performed a series of experiments using Cx3Cr1gfp/gfp and Ccr2−/− mice as controls. Indeed, CX3CR1 has been shown to regulate peripheral survival of patrolling monocytes but is devoid of chemotactic activity involved in BM egress. Reciprocally, CCR2 is essential for monocyte egress from the BM but is not involved in their survival. The distribution of Ly6C− monocytes in Cx3cr1gfp/gfp and Ccr2−/− mice is in fact very similar to that of S1pr5−/− mice, with a near normal frequency in the BM and a low frequency of these cells at the periphery (Fig. 4A).