# B7 Family Molecule VSIG4 Regulates Pulmonary Anti-Influenza Immune Responses via C-Type Lectin Signal Pathway

## TL;DR

VSIG4, a B7 family molecule, helps regulate immune responses to influenza by interacting with the C-type lectin receptor pathway, reducing inflammation and improving survival.

## Contribution

VSIG4's role in regulating acute influenza immune responses via the C-type lectin receptor pathway is newly identified.

## Key findings

- VSIG4-deficient mice showed reduced survival rates after influenza infection.
- VSIG4 interacts with CD209, a C-type lectin receptor, to regulate immune responses.
- VSIG4 inhibits excessive Th1 responses and inflammation during influenza.

## Abstract

Background: As the member of the B7 family, V-set and immunoglobulin domain-containing 4 (VSIG4) plays an essential role in regulating immune responses against bacterial infection, autoimmune disease, and chronic viral infection. However, the role of VSIG4 in acute viral infections remains largely unclear. Methods: Here, we constructed a gene-targeted VSIG4-deficient mouse model and then infected it with influenza to explore the detailed VSIG4-involved mechanism. Results: Our results demonstrated that the gene-deficient mice exhibited reduced survival rates, ranging from 25% to 50%, after being infected with different influenza virus strains. At the sites of infection, an increased number of CD8+ T cells, along with heightened expression of pro-inflammatory cytokines, e.g., Il-6 and TNFα, may have contributed to tissue damage. The recombinant VSIG4 protein slightly improved protection from the influenza challenge, suggesting regulatory functions of VSIG4 during infection. Using in vitro cell models, we show that the type C lectin receptor pathway member DC-SIGNR1 (CD209) is an essential factor during acute virus infection. The affinity and CO-IP tests indicated an interaction between CD209 and VSIG4, but not through protein modification. Conclusions: Therefore, VSIG4 functionally protected mice by regulating the type C lectin receptor pathway to inhibit excessive Th1 immune responses and inflammation. Our findings highlight the importance of considering immune homeostasis in the development of therapies for severe infections.