N N N

OLInked glycosyWiort domain

Cysteine region

FibrûnecïLn type m domains

Transmembrane domain

Protein-tyros no phosphatase domains

CD459 CD45e CD45ß CD45a CD45C CD45Y CD45r| CD455

Figure 2 Representation of the sizes of different CD45 isoforms compared to the T cell receptor (TCR). Estimates are based on either electromicroscopy data or Inferred from crystal structures.

Serine/

threonine cluster

Fibronectin type III repeats

] Transmembrane domain

I Tyrosine phosphatase

-1 domains

CD459 CD45e CD45ß CD45a CD45C CD45Y CD45r| CD455

Figure 2 Representation of the sizes of different CD45 isoforms compared to the T cell receptor (TCR). Estimates are based on either electromicroscopy data or Inferred from crystal structures.

complicates the characterization of isoform expression with these reagents. In addition, many of the CD45R antibodies are sensitive to carbohydrate modifications and some are specific for carbohydrate epitopes, not all of which are dependent on the presence of the alternatively spliced exons. While the CD45R antibodies provide only a limited insight into the specific patterns of isoform expression, they have proved a powerful tool in demonstrating that CD45 isoform expression is exquisitely regulated by different leukocyte populations.

T cell lineages express distinct CD45 isoforms depending on compartment and activation state. Most thymocytes express the CD456 isoform, however there is increased expression of the higher molecular weight isoforms during thymocyte development. Of interest, very early thymocyte precursors that do not express TCR, CD4 or CDS also stain for CD45R isoforms. This may indicate that thymocyte maturation is accompanied by cyclical expression of

Table 1 CD45 isoforms

Isoform

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