Signal regulatory protein molecules are differentially expressed by CD8- dendritic cells

A normalized subtracted gene expression library was generated from freshly isolated mouse dendritic cells (DC) of all subtypes, then used to construct cDNA microarrays. The gene expression profiles of the three splenic conventional DC (cDC) subsets were compared by microarray hybridization and two genes encoding signal regulatory protein beta (Sirp beta 1 and Sirp beta 4) molecules were identified as differentially expressed in CD8(-) cDC. Genomic sequence analysis revealed a third Sirp beta member localized in the same gene cluster. These Sirp beta genes encode cell surface molecules containing extracellular Ig domains and short intracytoplasmic domains that have a charged amino acid in the transmembrane region which can potentially interact with ITAM-bearing molecules to mediate signaling. Indeed, we demonstrated interactions between Sirp beta 1 and beta 2 with the ITAM-bearing signaling molecule Dap12. Real-time PCR analysis showed that all three Sirp beta genes were expressed by CD8(-) cDC, but not by CD8(+) cDC or plasmacytoid pre-DC. The related Sirp alpha gene showed a similar expression profile on cDC subtypes but was also expressed by plasmacytoid pre-DC. The differential expression of Sirpa and Sirp beta 1 molecules on DC was confirmed by staining with mAbs, including a new mAb recognizing Sirp beta 1. Cross-linking of Sirp beta 1 on DC resulted in a reduction in phagocytosis of Leishmania major parasites, but did not affect phagocytosis of latex beads, perhaps indicating that the regulation of phagocytosis by Sirp beta 1 is a ligand-dependent interaction. Thus, we postulate that the differential expression of these molecules may confer the ability to regulate the phagocytosis of particular ligands to CD8(-) cDC.