The processing and presentation of the self-antigen hemoglobin. Self-reactivity can be limited by antigen availability and costimulator expression.

APC do not distinguish between self- and foreign proteins. Previous studies from our laboratory demonstrated that most endogenous host APC constitutively processed and presented the self-Ag, hemoglobin (Hb), as detected by the Hb-specific T cell hybridoma, YO1.6. We have now examined APC in organs known to be involved in RBC degradation (liver Kupffer cells and splenic small resting B cells) for the presence of Hb/Ia complexes and for the expression of the costimulation necessary to trigger proliferation of T cell clones. We detected Hb/Ia complexes not only on splenic small resting B cells, but also on liver Kupffer cells. Interestingly, complexes were not present on lymph node small resting B cells. Splenic small resting B cells expressed costimulatory activity and efficiently stimulated the Th2 clones only. The opposite pattern was observed with liver Kupffer cells, which expressed costimulatory activity for Th1 clones only. However, if costimulatory activity was provided for the Th2 clones (IL-1 beta) and Th1 clones (allogenic spleen cells), the clones did proliferate in response to Kupffer cells and small resting B cells, respectively. In this report we have demonstrated that 1) endogenously formed self Hb/Ia complexes are expressed on splenic small resting B cells and liver Kupffer cells but not on lymph node small resting B cells and 2) these APC are also able to limit the expression of costimulatory activity for Th2 and Th1 T cell clones. Thus, endogenous APC not only constitutively process and present the self-Ag Hb, but also limit expression of the costimulatory activity necessary to trigger T cell proliferation against a self-Ag. The constitutive processing and presentation of self-Ag, as well as the regulation of costimulatory activity on APC, is likely an important feature of the maintenance of self-tolerance.