Neutrophils process exogenous bacteria via an alternate class I MHC processing pathway for presentation of peptides to T lymphocytes.

Peptides that are presented by class I MHC (MHC-I) molecules derive from cytosolic Ags processed via the conventional MHC-I pathway or exogenous Ags processed via alternate MHC-I processing mechanisms. Alternate MHC-I processing by macrophages and dendritic cells allows presentation of peptides from particulate Ags, including bacteria. Despite the established phagocytic activity of neutrophils, MHC-I processing and presentation of phagocytosed Ags by neutrophils has not been investigated. Murine neutrophils from peritoneal exudates were shown to express MHC-I molecules and tested for the ability to process HB101.Crl-OVA, Escherichia coli transfected to express a fusion protein containing the 257-264 epitope of OVA. Neutrophils were found to process HB101.Crl-OVA and present OVA(257-264)-K(b) complexes to CD8OVA T hybridoma cells via a pathway that was resistant to brefeldin A, an inhibitor of anterograde endoplasmic reticulum-Golgi transport, and lactacystin, a proteasome inhibitor. These results suggest that neutrophils process phagocytosed bacteria via a vacuolar alternate MHC-I pathway that does not involve cytosolic processing. In addition, neutrophils were found to secrete or "regurgitate" processed peptide that was subsequently presented by neighboring prefixed macrophages or dendritic cells. Thus, neutrophils may influence T cell responses to bacteria, either by directly presenting peptide-MHC-I complexes or by delivering peptides to other APCs for presentation. Hypothetically, neutrophils may directly present peptide to effector T cells in vivo at sites of inflammation, inducing cytokine production, whereas dendritic cells in receipt of neutrophil-derived antigenic peptides may migrate to lymphoid organs to initiate T cell responses.