The Role of Beta 2 Integrins in Macrophage Migration During Resolution of Inflammation.

Abstract 3600

Poster Board III-537

The mobilization of blood monocytes and their differentiation into macrophages during the immune-inflammatory response helps to prepare the tissue for resolution. During the resolution phase of inflammation macrophages do not die locally: some cells emigrate by draining lymphatics whereas some remain at the site of inflammation. The major myelo-monocytic integrin α_Mβ₂ (Mac-1, CD11b/CD18), together with two related integrins α_Dβ₂ (CD11d/CD18) and α_Xβ₂ (CD11c/CD18), mediate critical adhesive reactions of monocyte/macrophages. However, the roles of these adhesion receptors in control of macrophage retention at sites of inflammation and their emigration to lymph nodes are unclear. Using a mouse model of sterile peritonitis induced by thioglycollate injection, we examined the dynamics of macrophage β₂ integrins during the resolution phase of inflammation. Macrophages were defined by FACS analyses as a population of cells expressing α_Mβ₂^high, α_Dβ₂⁺ and CD115⁺. The initial population of resident β₂, positive for β_Dβ₂ and negative for α_Xβ₂. The thioglycollate-challenged mice showed a ∼4-fold increase in macrophages on day 3 followed by a progressive decrease to normal resident cell numbers by day 13. Expression of α_Mβ₂ on macrophages on day 3 decreased by 2.5-fold as a result of dilution of the initial population of α_Mβ₂^high resident macrophages by infiltrating blood monocytes expressing α_Mβ₂^low. However, after day 3, the density of α_Mβ₂ on macrophages gradually increased and by day 13 returned to the high levels characteristic of resident macrophages. By contrast, expression of α_Dβ₂ and α_Xβ₂ on inflammatory macrophages increased by 2-fold by day 6-9 compared to that on resident macrophages and then returned to the resident levels by day 3. Thus, although the number of macrophages decreased from day 3 to day 9 by several fold, the population of macrophages which remained in the peritoneum was enriched in cells expressing the high levels of α_Mβ₂ and α_Dα₂. Tracking migration of fluorescently labeled peritoneal cells demonstrated that a population of macrophages which leaves the inflamed peritoneum and enters lymph nodes consists of cells expressing low levels of α_Mβ₂ and α_Dβ₂. These data suggested that upregulation of β₂ integrins, especially α_Mβ₂, may be responsible for the retention of macrophages in the peritoneum. Indeed, the rate of macrophage emigration from the peritoneum in the α_Mβ₂-deficient mice was significantly higher than that in wild-type mice. The results indicate that macrophage emigration from the inflamed site is controlled by the level of integrin α_Mβ₂ and α_Dβ₂ with low expressors being migratory and high expressors remaining in the peritoneum. The data also highlight the importance of integrins α_Dβ₂ and α_Xβ₂ as specific markers of inflammatory macrophages.