CD45

CD45, also known as Protein tyrosin phosphatase, receptor C (PTPRC), and leucocyte common antigen (LCA), is a transmembrane glycoprotein with at least six human isoforms. They share a common intracellular domain and have heavily glycosylated extracellular domains. The isoforms are produced by alternative mRNA splicing of three exons (exon 4-6). CD45 has intrinsic tyrosine phosphatase activity, playing an important role in signal transduction, inhibition or upregulation of various immunological functions. The gene is located on chromosome 1q31-32. 

CD45 is expressed exclusively on cells of hematopoietic lineage with the exception of mature red cells. It is not detected on differentiated cells of other tissues. Almost all heematolymphoid cells, including precursor cells and mature B- and T-lymphocytes, granulocytes, monocytes/histiocytes and interdigitating reticulum cells and follicular dendritic cells express CD45. 

The isoforms are designated RABC, RAB, RBC, RB, RA and RO and are expressed on different subsets of hematopoietic cells. 

CD45 is detected in the large majority of hematolymphoid neoplasms. Overall, about 90% of malignant lymphomas are CD45 positive. The proportion is lower among precursor B-cell neoplasms (80% of B-ALL) and large cell anaplastic lymphomas, and only about 10% of plasmacytic neoplasms are positive. In Hodgkin lymphoma, the L&H cells in the LP-type are always positive, while Reed-Sternberg cells in classic Hodgkin lymphoma are negative or only show a faint cytoplasmic staining. CD45 is also detected in virtually all cases of mast cell neoplasms, and true histiocytic and dendritic cell neoplasms, with the possible exception of fibroblastic variants.

CD45 is an important marker in the primary tumour screening panel in order to identify hematolymphoid differentiation.

Tonsil and liver are recommendable as positive and negative tissue controls. In tonsil, all lymphocytes (B- and T-cells) and histiocytes must display a strong distinct membranous staining reaction. Squamous epithelial cells should be negative. In liver, the Kupffer cells should show a weak to moderate staining reaction whereas hepatocytes must be negative. Scattered lymphocytes can be seen in the liver and must display strong, distinct membranous staining reaction.

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