Substance P regulates natural killer cell interferon‐γ production and resistance to Pseudomonas aeruginosa infection

Studies have shown that after Pseudomonas aeruginosa (P. aeruginosa) corneal infection, BALB/c mice that are capable of resolving the disease, locally produce IFN‐γ. As T cells are not detected in the infected cornea of these mice, antibody depletion was used to test whether NK cells produce the cytokine. After depletion, decreased corneal IFN‐γ mRNA and increased disease severity, bacterial load, and PMN infiltrate resulted. Further work determined if substance P (SP), a pro‐inflammatory neuropeptide, participated in regulation of this response. To this end, mice were treated with the SP antagonist, spantide I that blocks SP interaction with neurokinin‐1, its major receptor. The treatment significantly decreased corneal IFN‐γ and IL‐18 protein levels and corneal perforation resulted. In vitro experiments using isolated splenic NK cells confirmed their ability to respond to IL‐18 and SP and to secrete IFN‐γ protein. We conclude: that for development of the BALB/c resistance response, NK cells are required to produce IFN‐γ; that the cells express the neurokinin‐1 receptor; and that SP directly regulates IFN‐γ production through this receptor. The data suggest a unique link between the nervous system and development of innate immunity in the cornea.