IL-21 protein is expressed in the GI tract and found in serum in recipient mice with xenogeneic GVHD. To determine whether IL-21 is produced by human cells in a xenogeneic model of GVHD, colon, small intestine, and blood were removed from NOD/SCID/γc−/− mice receiving 30 or 60 × 106 human PBMCs and assayed for IL-21. (A) Left: Representative large intestine from mice receiving 30 × 106 PBMCs, probed with anti–IL-21. Arrow points to an immunoreactive MNC. Right: Same tissue probed with a biotinylated Fc control. (B) Quantitative analysis comparing the degree of MNC infiltrate and the relative number of IL-21–secreting cells that are present in colon sections. N/E indicates not evaluable. (C) Comparison of the IL-21 concentration in sera with the number of CD4+ (C) or CD8+ (D) T cells present in the blood on day 10. Serum samples taken from animals receiving 30 or 60 × 106 PBMCs (open and filled symbols, respectively) on day 10 (diamonds), day 20 (squares), or when animals became moribund (circles).
