Antibodies (Abs) that mediate the hyperacute rejection and acute vascular rejection/delayed xenograft rejection of pig organs in humans and Old World primates are predominantly directed at a single carbohydrate epitope, galactose-α1,3-galactose (α1,3Gal). The T-cell dependence of elicited anti-α1,3Gal Ab responses in humans and Old World primates is controversial. In this study we have characterized anti-α1,3Gal Ab production in mice with disrupted α1,3-galactosyltransferase genes (GT-Ko mice) and determined the T-cell dependence of anti-α1,3Gal Ab responses, following xenograft and allograft transplantation. GT-Ko mice produce natural anti-α1,3Gal IgM and IgG in an age-dependent manner, however, these Abs could not elicit hyperacute rejection nor affect the rate of cardiac xenograft (3-5 days) or allograft rejection (7-9 days). Transplantation of xenogeneic Lewis rats hearts elicited modest anti-α1,3Gal Ab, but vigorous xenoAb responses. The anti-α1,3Gal Ab response was restricted to the IgM and IgG3 subclass while the xenoAb response comprised IgM and all four IgG subclasses. Transplantation of allogeneic C3H hearts elicited weak anti-α1,3Gal Ab responses that were primarily IgM, but vigorous alloAb responses. Despite the restriction of elicited anti-α1,3Gal Ab responses to the IgM and IgG3 isotypes, these responses are T-cell dependent. The ability of allografts to elicit weak anti-α1,3Gal but strong allo-Ab responses, can be explained by the dependence of α1,3Gal-specific B cells on cognate help from T cells. (C) 2000 Elsevier Science S.A. All rights reserved.