Aberrant TCR-mediated signaling in CD45-null thymocytes involves dysfunctional regulation of Lck, Fyn, TCR-zeta, and ZAP-70.

Abstract

CD45 is a transmembrane phosphotyrosine phosphatase expressed on all nucleated hemopoietic cells. Targeting of CD45 exon 9 has generated a mouse line completely lacking CD45 expression (CD45-null) in which there are severe abnormalities in T cell development. Defects in TCR-mediated signals underlying these abnormalities have now been investigated using CD45-null T cells. No T cell proliferation was detected in response to a CD3 mAb. In thymocytes the p56(lck) and p59(fyn) tyrosine kinases were hyperphosphorylated, and p56(lck) was in its inactive conformation. Both basal and TCR-stimulated tyrosine phosphorylation of TCR-zeta and CD3-epsilon were much reduced, and TCR stimulation induced an abnormal p18 phosphoisomer of TCR-zeta previously noted in T cells stimulated by altered peptide ligands. These defects were associated with the failure of ZAP-70 kinase recruitment to the TCR-zeta chain. TCR coupling to the tyrosine phosphorylation of several proteins, including HS1 and p120(cbl), was also much reduced. However, TCR-induced signaling was not ablated, and significant inositol phosphate and calcium signals were observed in CD45-null thymocytes. Our molecular analysis suggests that the threshold for TCR signal transduction is greatly increased in CD45-null T cells, thus explaining the profound defects in thymic development.

T cell signaling effect of age

“…Studies on CD45-deficient mice have shown that in the absence of this phosphatase, TCR signaling is markedly impaired as thymocytes in CD45 -/mice proliferated less in response to stimulation (87)(88)(89). One of the proposed functions of CD45 in TCR signaling is to activate the Srcfamily kinases (Lck and Fyn) by dephosphorylating these tyrosine kinases (90). Data from studying T cells in CD45 -/mice have shown that Lck and Fyn were inactive due to a high level of phosphoryation (89).…”

Regulation of CD45 phosphatase by oncogenic ALK in anaplastic large cell lymphoma

“…Therefore, CD45 has become a diagnostic marker to distinguish between different T cells because naïve T cells express the isoform CD45RA, while memory and activated T cells express CD45RO (12). Besides TCR regulation through the Src-family kinase Lck, CD45 is a regulator of cytokine activation through the negative regulation of the JAK/STAT pathway (12)(13)(14). Indeed, CD45 directly dephosphorylates all the JAK family members resulting in decreased JAK activity and, consequently, affecting STAT transcription factors (15).…”

CD45 limits Natural Killer cell development from common lymphoid progenitors

“…CD45 has the ability to influence multiple signalling pathways through its various biochemical functions, the dominant one being dephosphorylation of the C-terminal negative regulatory tyrosine in the Src-family kinases, consequently activating Src kinases upon T and B cell receptor (TCR; BCR) engagement [4][5][6]16]. It is therefore an indispensable positive regulator of T cell antigen receptor signalling in cell lines, mouse models and humans [17][18][19][20].…”

“…In addition to its role in TCR and BCR activation [4][5][6]16], CD45 has been reported to activate Syk, JNK and p38, downstream of Ly49D activation in NK cells [2,25]. Lastly, CD45 has been shown to suppress Janus kinase (JAK) activity, resulting in negative regulation of cytokine receptor signalling [26].…”

“…In mice, the Ptprc gene consists of 34 exons that encodes an extracellular domain (exons 1-12), interdomain (exons [13][14], transmembrane domain (exon 15), and an intracellular cytoplasmic domain (exons [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. T, B and NK cells express multiple CD45 isoforms, that are differentially expressed during developmental stages [10].…”