Expression of the low Mr isoform of CD45 (CD45RO) in B-cell non-Hodgkin's lymphomas

The CD45 antigen family consists of multiple molecular isoforms ranging from 180 to 220 relative molecular mass (Mr). The highest Mr isoforms are recognized by monoclonal antibodies (MoAbs) designated CD45RA, while those recognizing the low Mr isoform are designated CD45RO. About half of the T-cells in peripheral blood express CD45RA while the remainder express CD45RO. A switch from the high to the low Mr isoform of CD45 has been found in association with the process of T-cell stimulation and acquisition of "memory." B-cells normally express CD45RA, but not CD45RO. However, under stimulatory conditions, B-cells may be capable of undergoing an isoform switch and expressing CD45RO. The expression of this low Mr isoform of CD45 was investigated in lymphomas composed of monoclonal B-cells to determine if such a switch occurs in malignant B-cell populations. The vast majority (110/117 cases) of B-cell lymphomas expressed only CD45RA, while a very small number (7/117 cases) expressed CD45RO, but not CD45RA. There was no relationship between the CD45RO expression and the histologic subtype. The physiological significance of this unusual expression of CD45RO in a subpopulation of B-cell lymphomas is not clear. In that CD45RO, as defined by the MoAb UCHL 1, is typically used as a marker of T-cells in tissue sections, caution must be exercised in interpretation, since not all T-cells are reactive and some B-cell lymphomas are reactive.