Activation threshold is lower in atypical cells than in naive cells.

CD4⁺ T cells were separated, and naive and atypical subsets were sorted from 50 mL blood. Proliferation was assessed in response to PHA (0.01-10 μg/mL; Sigma) or anti-CD3 (OKT3, 0.01-1 μg/mL) with or without anti-CD28 (YTH913.12, 5 μg/mL) antibodies. Proliferation was quantified by the incorporation of ³H-thymidine (1 μCi/well) after 5 days of culture and was normalized to cells in medium only. Results represent the means and SD of independent experiments using 5 RA patients.