Abstract
CD84 is a self-binding receptor from the CD150 (or signaling lymphocyte activation molecule [SLAM]) family that is broadly expressed in hematopoietic cells. It has been described that the adaptors SLAM-associated protein (SAP) and EWS-FLI1-activated transcript 2 (EAT-2) are critical for CD150 family members' signaling and function. We observed that human mast cells express CD84 but lack SAP or EAT-2, that CD84 is tyrosine phosphorylated upon FcεRI engagement, and that the release of granule contents is reduced when FcεRI is coengaged with CD84 in LAD2 and human CD34(+)-derived mast cells. In addition, we observed that the release of IL-8 and GM-CSF was also reduced in FcεRI/CD84-costimulated cells as compared with FcεRI/Ig control. To understand how CD84 downregulates FcεRI-mediated function, we analyzed signaling pathways affected by CD84 in human mast cells. Our results showed that CD84 dampens FcεRI-mediated calcium mobilization after its co-cross-linking with the receptor. Furthermore, FcεRI-mediated Syk-linker for activation of T cells-phospholipase C-γ1 axis activity is downregulated after CD84 stimulation, compared with FcεRI/Ig control. The inhibitory kinase Fes phosphorylates mainly the inhibitory motif for CD84. Moreover, Fes, which has been described to become phosphorylated after substrate binding, also gets phosphorylated when coexpressed with CD84. Consistently, Fes was observed to be more phosphorylated after CD84 and FcεRI co-cross-linking. The phosphorylation of the protein phosphatase Src homology region 2 domain-containing phosphatase-1 also increases after CD84 and FcεRI coengagement. Taken together, our results show that CD84 is highly expressed in mast cells and that it contributes to the regulation of FcεRI signaling in SAP- and EAT-2-independent and Fes- and Src homology region 2 domain-containing phosphatase-1-dependent mechanisms.