The receptor-type protein tyrosine phosphatase (RPTP) CD148 is thought to have an inhibitory function in signaling and proliferation in nonhematopoietic cells. However, its role in the immune system has not been thoroughly studied. Our analysis of CD148 loss-of-function mice showed that CD148 has a positive regulatory function in B cells and macrophages, similar to the role of CD45 as a positive regulator of Src family kinases (SFKs). Analysis of CD148 and CD45 doubly deficient B cells and macrophages revealed hyperphosphorylation of the C-terminal inhibitory tyrosine of SFKs accompanied by substantial alterations in B and myeloid lineage development and defective immunoreceptor signaling. Because these findings suggest the C-terminal tyrosine of SFKs is a common substrate for both CD148 and CD45 phosphatases and imply a level of redundancy not previously appreciated, a reassessment of the function of CD45 in the B and myeloid lineages based on prior data from the CD45-deficient mouse is warranted.

B1 and B2 B cells differ in their ability to respond to T-cell-independent (TI) antigens. Here we report that the Src-family kinase (SFK) regulator CD148 has a unique and critical role in the initiation of B1 but not B2 cell antigen receptor signaling. CD148 loss-of-function mice were found to have defective B1 B-cell-mediated antibody responses against the T-cell-independent antigens NP-ficoll and Pneumovax 23 and had impaired selection of the B1 B cell receptor (BCR) repertoire. These deficiencies were associated with a decreased ability of B1 B cells to induce BCR signaling downstream of the SFK Lyn. Notably, Lyn appeared to be selectively regulated by CD148 and loss of this SFK resulted in opposite signaling phenotypes in B1 and B2 B cells. These findings reveal that the function and regulation of Lyn during B1 cell BCR signaling is distinct from other B cell subsets.

Neutrophils, critical innate immune effectors, use bacterial-derived chemoattractant-induced G protein-coupled receptor (GPCR) signaling for their pursuit of bacteria. Tyrosine phosphorylation pathways and receptor-like tyrosine phosphatases (RPTPs) are rarely considered in chemoattractant-mediated GPCR signaling. Here, we report that two RPTPs, CD45 and CD148, previously shown to share redundant roles in positively regulating Src family kinases (SFKs) in immunoreceptor signaling pathways in B cells and macrophages, are critical in the neutrophil response to S. aureus infection and, surprisingly, in chemoattractant-mediated chemotaxis. Remarkably, deficiency in either of these RPTPs influenced neutrophil GPCR responses in unique ways. Our results reveal that CD45 positively while CD148 positively and negatively regulate GPCR function and proximal signals including Ca(2+), phosphatidylinositol 3'OH kinase (PI3K), and phospho-extracellular regulated kinase (pERK) activity. Moreover, our results suggest that CD45 and CD148 preferentially target different SFK members (Hck and Fgr versus Lyn, respectively) to positively and negatively regulate GPCR pathways.

The autoimmune regulator (Aire) is essential for prevention of autoimmunity; its role is best understood in the thymus, where it promotes self-tolerance through tissue-specific antigen (TSA) expression. Recently, extrathymic Aire-expressing cells (eTACs) have been described in murine secondary lymphoid organs, but the identity of such cells and their role in immune tolerance remains unclear. Here we have shown that eTACs are a discrete major histocompatibility complex class II (MHC II)(hi), CD80(lo), CD86(lo), epithelial cell adhesion molecule (EpCAM)(hi), CD45(lo) bone marrow-derived peripheral antigen-presenting cell (APC) population. We also have demonstrated that eTACs can functionally inactivate CD4⁺ T cells through a mechanism that does not require regulatory T cells (Treg) and is resistant to innate inflammatory stimuli. Together, these findings further define eTACs as a distinct tolerogenic cell population in secondary lymphoid organs.

The kinase-phosphatase pair Csk and CD45 reciprocally regulate phosphorylation of the inhibitory tyrosine of the Src family kinases Lck and Fyn. T cell receptor (TCR) signaling and thymic development require CD45 expression but proceed constitutively in the absence of Csk. Here, we show that relative titration of CD45 and Csk expression reveals distinct regulation of basal and inducible TCR signaling during thymic development. Low CD45 expression is sufficient to rescue inducible TCR signaling and positive selection, whereas high expression is required to reconstitute basal TCR signaling and beta selection. CD45 has a dual positive and negative regulatory role during inducible but not basal TCR signaling. By contrast, Csk titration regulates basal but not inducible signaling. High physiologic expression of CD45 is thus required for two reasons-to downmodulate inducible TCR signaling during positive selection and to counteract Csk during basal TCR signaling.