Changes in Macrophage Populations: Phenotypic Differences between Normal and Tumor-Bearing Host Macro phages

Abstract

As a tumor grows, changes occur in the function of macrophages (MΦ). This is concomitant with changes in their phenotype. Flow cytometric analysis of monoclonal antibody (mAb)labeled thioglycollate-elicited peritoneal, and resident splenic, MΦ showed a tumor-induced shift of Mac-1, -2, -3, and Ia antigen expression. During tumor growth, the percentage of peritoneal Mac-2 +, -3 +, and Ia + MΦ decreased significantly (22 %, 14%, and 58%, respectively), while Mac-1 + MΦ remained unchanged. By analyzing the data on two-dimensional histograms and comparing the sizes of MΦ to cell-surface antigen expression, we identified distinct subpopulations of peritoneal MΦ. Three distinct size versus antigen expression MΦ subpopulations were detected by flow cytometry and consisted of 10-16, 17-22, and 23-27 microns for the small-, medium-, and large-sized populations, respectively. Large-sized Mac-1 + and -2 + MΦ decreased (37% and 38%), while large-sized Mac-3 + MΦ did not decrease during tumor growth. Medium-sized Mac-3 + MΦ decreased 33% during tumor growth, while no differences could be seen in medium-sized Mac-1 + or -2 + MΦ. Concomitant with the decrease in large-sized Mac-1 + MΦ was an increase in small-sized Mac-1 + MΦ. Peritoneal Ia + MΦ were mostly small-sized (4-7-fold increase over the medium-sized and none in the largesized population). MΦ Ia antigen expression was nearly absent in the 21-day tumor-bearing host, with < 4% of the cells labeling positive (a 73% drop from normal host MΦ). In splenic MΦ, the percentage of Mac-1 + MΦ significantly increased (90 %) during tumor growth, while Mac-2 + and -3 + MΦ showed a smaller, but still significant, increase (48% and 40%, respectively). Additionally, splenic Ia + MΦ significantly decreased (29%) during tumor growth. More important than the decreased cell numbers was the significant decrease in Ia antigen expression per cell. Unlike the peritoneal MΦ, the splenic MΦ did not show distinct size versus antigen expression subpopulations, although there was an overall difference in MΦ size between normal and TBH. These data suggested that MΦ from different anatomical sites are phenotypically different and tumor growth mediates phenotypic alterations in peritoneal and splenic MΦ populations. This may be the source of tumor-induced dysfunction of MΦmediated immune activity.