Splenic CD11b⁺Gr-1+ cells from mice with progressing tumors consist of a monocytic and a granulocytic fraction. Splenocytes from BW-Sp3 tumor-bearing mice with progressing tumors (average tumor diameter ± 20 mm), 5 to 7 weeks after inoculation, were labeled with PE- or FITC-conjugated mAbs recognizing surface antigens for FACS analysis. (A) Dot plots representing costainings of PE-labeled anti-CD11b with FITC-labeled anti–Gr-1, anti-Ly6G, or anti-Ly6C. To detect 7/4 expression, cells were incubated with purified anti-7/4 mAb, followed by PE-labeled anti–rat IgG. Isotype-matched control mAbs did not significantly stain the cells (data not shown). (B) SSC profiles of cells gated in regions R1 (dotted line) versus R2 (full line); R3 (dotted line) versus R4 (full line); and R5 (dotted line) versus R6 (full line). (C) Splenocytes were coincubated with FITC-labeled anti–Gr-1 and PE-labeled anti-F4/80. To detect M-CSFR expression, cells were incubated with purified anti–M-CSFR mAb, followed by PE-labeled anti–rat IgG. Histograms represent isotype (dotted line) and antigen-specific (full line) staining on the gated Gr-1^int and Gr-1^hi populations. Profiles are representative of 1 progressor spleen of at least 5 investigated. (D) Six weeks after tumor inoculation, a subdivision was made between mice that rejected their tumor (regressors), mice with a short history of tumor progression and an average tumor diameter 15 mm or smaller (early progressors), or mice with a longer history of tumor progression and average tumor diameter of more than 15 mm (late progressors) before death. Tumor growth curves of individual mice in each group are shown. Splenocytes from a minimal of 5 mice in each group were stained with PE-labeled anti-CD11b and FITC-labeled anti–Gr-1. A dot plot from one mouse, representative for each group, is given. Average percentages ± SD of Gr-1^int and Gr-1^hi cells within the total splenocyte population are shown for each group.