Volumetric and functional heterogeneity of human monocytes

Volume analysis of purified human blood monocytes revealed distinct populations of large and small cells. Computer curve fitting suggested a third, intermediate-sized population. These monocytes were designated M1, M2, and M3 in order of increasing size, and their approximate volumes were 150, 250, and 480 micron3, respectively. The three subpopulations were present in all 30 normal individuals tested. Two new techniques were developed that separate monocytes into M1 + M2 and M3 fractions; one used preferential incorporation of carbonyl iron particles by M3 cells and the other used the selective aggregation of M3 cells by thrombin in the presence of platelets. The chemotactic response to zymosan-activated human serum by total monocytes, M1 + M2 monocytes, and M3 monocytes was determined by the agarose plate method. In all experiments M3 monocytes were 10-fold more responsive than M1 + M2 monocytes and were significantly more so than total monocytes. These findings suggest that M3 cells are the major subpopulation capable of directional migration. This investigation establishes the existence of volumetrically definable subpopulations of human monocytes that are functionally distinct.