CD45Expression

CD45, also known as protein tyrosine phosphatase receptor type C (PTPRC), is a transmembrane protein tyrosine phosphatase expressed on the surface of most nucleated hematopoietic cells. It is involved in regulating antigen receptor signaling in lymphocytes and other immune cells. CD45 is found on T cells, B cells, natural killer cells, monocytes, granulocytes, and dendritic cells, but not on mature erythrocytes or platelets, reflecting its association with hematopoietic lineages.

The extracellular domain of CD45 contains variable regions generated by alternative splicing of exons A, B, and C, producing several isoforms. The main isoforms include CD45RA, CD45RB, CD45RC, and CD45RO, with CD45RO lacking the A, B, and C exons. Expression of these isoforms correlates with cell type and activation state: naive T cells commonly express CD45RA, while memory and previously activated T cells often express CD45RO. B cells and other myeloid cells exhibit their own isoform patterns, and CD45 expression is frequently used as a marker in immunophenotyping.

Functionally, CD45 modulates signaling thresholds by dephosphorylating Src family kinases (such as Lck in T cells and Lyn in B cells), thereby regulating downstream signaling through the T cell receptor (TCR) and B cell receptor (BCR). This regulation influences cell activation, differentiation, and development, including thymic selection and peripheral immune responses.

Clinical relevance lies in the diagnostic utility of CD45 patterns in flow cytometry, where aberrant expression can aid in identifying hematologic malignancies and delineating leukocyte lineages. While primarily a diagnostic marker, CD45’s role in immune signaling also informs research into immune regulation and targeted therapies.