Increased islet allograft survival after extended culture by a mechanism other than depletion of donor APCs : lack of correlation between the elimination of donor MHC class II-positive accessory cells and increased transplantability

Neonatal rat islets derived by nonenzymic (in vitro) isolation procedures from the Fischer-344 (F-344, Rt1lv1) strain have been shown to be freely transplantable across complete MHC barriers without the use of any form of immunosuppression. However, islets obtained by in vitro isolation from some donor strains, such as the ACI, retain a degree of immunogenicity upon transplantation. This study examined the immunogenic nature of these neonatal ACI islets, and correlated this with the presence of a residual population of MHC class II-positive antigen presenting cells within the islets. In addition, further reduction of immunogenicity of isolated neonatal ACI islets by extension of the culture period was examined. Islets were obtained from neonatal ACI donors by culture-isolation and placed in secondary culture for either 2 days (10 day total culture period, standard culture) or 42 days (50 day total culture period, extended culture). Standard-culture ACI islets were rejected 33% to 100% of the time in five different recipient strains (depending on recipient strain). Compared with control grafts of fresh pancreatic fragments, rejection rates were significantly lower in two strain combinations (ACI to BUF, P = 0.044 and ACI to F-344, P = 0.028), and for all recipient strains combined (P less than 0.001). Extended-culture ACI islets survived significantly longer in all five recipient strains tested (ACI to BN, P = 0.024; ACI to BUF, P = 0.005; ACI to LEW, P = 0.004; ACI to F-344, P = 0.008 and ACI to WF, P = 0.024) and for all strains combined (P less than 0.001). No MHC class II-positive cells were detected upon examination of sectioned neonatal ACI islets using peroxidase immunocytochemistry (OX-6 antigen) in either the standard-culture or extended-culture groups. These results suggest that the increased survival of grafts of extended culture ACI islets is due to a mechanism other than the elimination of class II-positive APCs from the islets. Although possible down-regulation of MHC class II expression on residual APCs during the culture process may make identification of these cells difficult, alternative mechanisms such as the down-regulation of MHC class I antigen, resulting in the reduction of available donor target, or the existence of an accessory cell that is not constitutively MHC class II-positive, may be responsible for this increased graft survival.