The influence of curli, a MHC-I-binding bacterial surface structure, on macrophage–T cell interactions

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Abstract
Escherichia coli express thin surface fimbriae called curli which bind soluble matrix proteins and major histocompatibility complex (MHC)-I molecules. The present study addressed the ability of purified curli or curliated E. coli to influence peptide presentation on MHC-I, T cell proliferation and bacterial uptake by macrophages. In vitro studies with curli-proficient E. coli YMel and the isogenic curli-deficient strain YMel-1, both expressing the model antigen Crl—OVA, showed that curli expression by E. coli does not appear to influence the efficiency by which the bacteria are processed by murine macrophages for OVA(257–264) presentation on Kb. Furthermore, curli expression by E. coli did not influence the binding of exogenously added OVA(257–264) peptide to Kb on the surface of prefixed macrophages. In addition, neither curliated nor non-curliated heat-killed bacteria influenced proliferation of either murine or human T cells stimulated with anti-CD3. Finally, curliated E. coli adhered to and were internalized by macrophages from C57BL/6 and MHC-I-deficient TAP1−/− mice equally well. Together these studies show that curli expression by E. coli does not appear to influence phagocytic processing of bacteria expressing Crl—OVA for OVA(257–264)/Kb presentation, the binding of exogenously added OVA(257–264) to Kb or T cell proliferation. In addition, although curli expression by E. coli enhances bacterial interaction with macrophages, curli interaction with MHC-I does not significantly contribute to this adherence.