ACTIVATION OF HUMAN CD4(-CELLS USING B-CELLS AS ACCESSORY CELLS()45RA(+) T)

Human naive CD4(+) T cells, as defined by expression of CD45RA and lac k of CD45R0, can be activated in vitro using B cells as accessory cell s. CD4(+)CD45RA(+) T cells proliferate, as determined by [H-3]thymidin e or bromodeoxyuridine (BrdU) incorporation, after activation with the superantigen staphylococcal enterotoxin A (SEA) presented by major hi stocompatibility complex class II-expressing B cells. The identity of the responding cells as being CD45RA(+) and not contaminating CD45R0() T cells was determined by FACS analysis, showing that purified CD45R A-expressing T-helper cells went into S phase and progressively acquir ed expression of the CD45R0 isoform while simultaneously losing expres sion of the CD45RA isoform. Cultivation of the CD4(+) T-cell subsets u nder limiting dilution conditions supported these findings and reveale d that (i) the frequency of responding cells in the CD45RA(+) populati on was equal to or higher than in the CD45R0(+) subset and (ii) that t he number of CD45R0(+) cells possibly contaminating the CD45RA populat ion was too low to be able to account for the response observed.