Establishment of a purified cell model of GVHD. Lethally irradiated B6D2F₁ recipients reconstituted with B6-derived HSCs (n = 8) or with HSCs plus 200 000 αβTCR⁺ splenocytes from B6 donors (n = 14) were assessed for evidence of acute GVHD. Mean ± SE clinical GVHD scores are displayed for each week after BMT. The recipients of HSCs alone showed no clinical GVHD. In contrast, recipients of 2000 HSC + 200 000 αβTCR⁺ donor T_SP exhibited severe and often lethal GVHD, with the peak severity and incidence occurring at week 3 (P = .0005 vs HSCs alone).