CD4 and systemic sclerosis: Topo I-reactive CD27+ B cells with low affinity and high affinity for topo I (100 cells each) and control topo I-non-reactive CD27+ B cells (100 cells) were determined from the peripheral blood of anti-topo I antibody-positive SSc patients and co-cultured with 104 CD4+ T cells on a 96-well plate for 48 hr.