The inhibitory receptor LILRB4 (ILT3) modulates antigen presenting cell phenotype and, along with LILRB2 (ILT4), is upregulated in response to Salmonella infection.

Background: Leukocyte Ig-like receptors (LILR) are a family of innate immune receptors with immunomodulatory functions. High-level expression of the receptors LILRB2 (ILT4) and LILRB4 (ILT3) is a feature of tolerogenic antigen presenting cells and has been observed in cancer and transplant situations. There are relatively few studies regarding these receptors in the context of infection and it is not yet clear how LILRB4 exerts its inhibitory effects.

Results: We studied the effects of LILRB4 ligation on antigen presenting cell phenotype, and the expression of LILRB2 and LILRB4 on Salmonella-infected antigen presenting cells. Ligation of LILRB4 throughout in vitro culture of dendritic cells led to an upregulation of the co-stimulatory protein CD86. Alterations in the production of IL-8 and IL-10 by LILRB4-ligated macrophages were also observed. Infection with Salmonella typhimurium or TLR stimulation with Salmonella components led to an upregulation of LILRB2 and LILRB4.

Conclusion: Our results indicate that the inhibitory effects of LILRB4 do not result from a failure to upregulate co-stimulatory proteins. In addition to the high level expression that can render antigen presenting cells tolerogenic, there may be a role for lower level expression and activity of LILRB2 and LILRB4 in response to TLR signalling during an immune response to bacterial infection.