C3aR Signaling Inhibits NK-cell Infiltration into the Tumor Microenvironment in Mouse Models

Many solid tumors have low levels of cytotoxic CD56^dim natural killer (NK) cells, suggesting that CD56^dim NK-cell exclusion from the tumor microenvironment (TME) contributes to the decreased response rate of immunotherapy. Complement component 3a (C3a) is known for its tumor-promoting and immunosuppressive roles in solid tumors. Previous reports have implicated the involvement of the C3a receptor (C3aR) in immune cell trafficking into the TME. C3aR is predominantly expressed on the surface of activated cytotoxic NK cells, but a specific role for C3aR in NK-cell biology has not been investigated. Because solid tumors generate elevated C3a and have decreased NK-cell infiltration, we hypothesized that C3aR might play a role in cytotoxic NK-cell recruitment into the TME. Our results indicate that blocking C3aR signaling in NK cells increased NK-cell infiltration into the TME in mouse models and led to tumor regression. Because the critical lymphocyte trafficking integrin LFA-1 orchestrates the migration of activated NK cells, we wanted to gain insight into the interaction between C3aR signaling and LFA-1. Our results demonstrated that direct interaction between C3aR and LFA-1, which led to a high-affinity LFA-1 conformation, decreased NK-cell infiltration into the TME. We propose that approaches to enhance cytotoxic NK-cell infiltration into the TME, through either disrupting C3a and C3aR interaction or inhibiting the formation of high-affinity LFA-1, represent a new strategy to improve the efficiency of immunotherapy for cancer treatment.

A statin-based inhibitor of lymphocyte function antigen-1 protects against ischemia/reperfusion-induced leukocyte adhesion in the colon

1. Statins are mainly used to control hypercholesterolemia; however, recent studies have also ascribed anti-inflammatory effects to the statins. LFA703 is a novel statin-derived compound, which potently inhibits lymphocyte function antigen-1 (LFA-1, CD11a/CD18) but does not affect HMG-CoA reductase activity. 2. The objective of this study was to examine the anti-inflammatory mechanisms of LFA703 in ischemia/reperfusion (I/R)-induced leukocyte-endothelium interactions in the colon. For this purpose, the superior mesenteric artery was occluded for 30 min and leukocyte responses were analyzed in colonic venules after 120 min of reperfusion in mice using inverted intravital fluorescence microscopy. 3. First, the inhibitory mechanisms of LFA703 on leukocyte adhesion were investigated in vitro using a mouse CD4+8+ thymocyte cell line. Immunoneutralization of LFA-1 and ICAM-1 abolished leukocyte adhesion, whereas inhibition of VLA-4 had no effect in this in vitro assay. Indeed, it was found that LFA703 dose-dependently reduced LFA-1-dependent leukocyte adhesion to mouse endothelial cells in vitro with an IC50 of 3.2 microm. 4. I/R caused an increase in leukocyte rolling and adhesion in colonic venules. Immunoneutralization of LFA-1 significantly reduced I/R-induced leukocyte adhesion by 89% in colonic venules. In contrast, I/R-provoked leukocyte rolling was insensitive to inhibition of LFA-1 function. 5. Administration of 30 mg kg-1 of LFA703 decreased reperfusion-induced leukocyte adhesion by more than 91%, while the level of leukocyte rolling was unchanged, suggesting that LFA703 effectively blocked LFA-1-dependent firm adhesion of leukocyte in the colon. However, LFA703 did not decrease the expression of LFA-1 on circulating leukocytes. 6. This study demonstrates that LFA-1 is indeed a critical adhesion molecule in mediating postischemic leukocyte adhesion in the colon. Moreover, this is the first study showing that a statin-based synthetic compound has the capacity to abolish LFA-1-dependent leukocyte adhesion in I/R. These novel findings may have great implications in the clinical treatment of conditions associated with I/R-induced tissue injury, such as organ transplantation, trauma and major surgery.