Activation of human CD4+45RA+ T cells using B cells as accessory cells.

Human naive CD4+ T cells, as defined by expression of CD45RA and lack of CD45R0, can be activated in vitro using B cells as accessory cells. CD4+CD45RA+ T cells proliferate, as determined by [3H]thymidine or bromodeoxyuridine (BrdU) incorporation, after activation with the superantigen staphylococcal enterotoxin A (SEA) presented by major histocompatibility 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 CD45RA-expressing T-helper cells went into S phase and progressively acquired expression of the CD45R0 isoform while simultaneously losing expression of the CD45RA isoform. Cultivation of the CD4+ T-cell subsets under limiting dilution conditions supported these findings and revealed that (i) the frequency of responding cells in the CD45RA+ population was equal to or higher than in the CD45R0+ subset and (ii) that the number of CD45R0+ cells possibly contaminating the CD45RA population was too low to be able to account for the response observed.