It has recently been recognized that obesity, a critical factor contributing to the number of abnormalities constituting the metabolic syndrome, is associated with adipose tissue inflammation which is attributable in large part to the increased numbers of infiltrating macrophages. Integrins and, in particular, the major myeloid-specific integrin αMβ2 (Mac-1) plays an essential role in leukocyte migration. We have found that during the period of 12-36 weeks, Mac-1-/- mice gained more weight on normal chow diet than control mice, which was due to the increase of white fat mass (∼1.7- and 3.3-fold for males and females, respectively) and not a result of increased food intake. To gain insight into the mechanisms that regulate fat storage in Mac-1-/- mice, we determined the number of macrophages present in omental fat band (OFB) which, similar to other fat depots, underwent the mass increase. Unexpectedly, the number of macrophages per mg of OFB was less in Mac-1-/- mice than that in wild-type mice at all ages examined with the largest difference observed at 6 weeks (∼2.9- and 2.5-fold for males and females, respectively), implicating Mac-1 in the control of macrophage influx and/or retention within OFB. To examine the role of Mac-1 in migration, resident and thioglycollate-elicited inflammatory macrophages were isolated from the peritoneum, labeled with PKH-26 fluorescent dye and their migration to and from OFB were determined by measuring tissue fluorescence. For the “walk-in” experiments, OFB was made “empty” by incubating the tissue for 24 h in a cell culture medium to release OFB-associated macrophages and lymphocytes. “Walk-out” experiments were performed by first filling OFB with labeled macrophages and then tracking their emigration from the tissue. The analyses of the kinetics of macrophage migration showed that both resident and inflammatory Mac-1-/- macrophages arrive to and exit from OFB faster than their wild-type counterparts. For example, the number of Mac-1-/- resident and inflammatory macrophages that accumulated in OFB after 4 h was 11±2.8 and 2.4±0.38 -fold higher than that of wild-type cells. Likewise, the rate of emigration from OFB for both resident and inflammatory was higher for Mac-/- macrophages than that of wild-type cells (4.4±0.55 and 2.8±0.15 after 6 h for inflammatory and resident macrophages, respectively). The distinction in migration was not due to the differences in cell adhesive properties as wild-type and Mac-/- macrophages adhered to the same extent to plastic dishes. To examine macrophage migration in vivo, PKH-26-labeled wild-type and Mac-/- macrophages were injected into the peritoneum of wild-type mice and their migration to and emigration from OFB was tracked by determining the fluorescence intensity of OFB excised after selected time points. Similar to their behavior in vitro, both resident and inflammatory Mac-/- macrophages migrated faster to and emigrated from OFB than wild-type cells. In addition, after the arrival to OFB, wild-type macrophages organized in large clusters, a pattern distinct from Mac-/- cells which were scattered on the surface and within the tissue. These results suggest that the reduced number of macrophages observed in OFB of Mac-/- mice may be due to their enhanced mobility and reduced capacity to establish adhesive bonds within the OFB tissue resulting in the reduced retention and increased turnover. Since less macrophages is present in fat in the overweight Mac-/- mice than in normal wild-type mice, the data also indicate that the increase in body fat can be dissociated from the increase in macrophage numbers in fat.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.