In vivo upregulation of CD45RA in neutrophils of acutely infected patients.

The CD45 family of high relative molecular mass (M(r)) cell surface antigens is expressed on all lymphohematopoietic cells. Different cell types express various M(r) isoforms in a precisely programmed fashion. The cytoplasmic domain of the CD45 family possesses protein tyrosine phosphatase activity and is involved in transmembrane signaling. Under normal conditions, the major CD45 isoform on neutrophils is CD45RO, the low M(r) isoform. We recently demonstrated a cytoplasmic granule-associated pool of CD45RA, the highest M(r) isoform, in mature neutrophils that is generally absent from the cell surface under nonstimulatory conditions. Calcium ionophore A23187 induced translocation of cytoplasmic CD45RA to the cell surface and increased the cell surface expression of CD45RO and CD45. The present study demonstrates that without in vitro stimulation, there is an increase in expression of CD45RA on the surface of neutrophils from patients with systemic, acute infections, while those of patients with localized infections are only slightly increased compared to normal controls. Although the exact functions of the various isoforms of CD45 are not known, this increased expression of CD45RA suggests a role for this high M(r) isoform during in vitro neutrophil activation.