CD44ICD promotes the fusion of macrophages and the differentiation of TRAP⁺multinucleated osteoclasts. (A) Rat alveolar macrophages were transduced with the retroviral vector MigR1 empty or encoding CD44FL, CD44ΔE, or CD44ICD, plated in fusogenic conditions, and then cultured for only 2 days. Whereas macrophages transduced with empty vector, CD44FL, or CD44ΔE had barely initiated fusion by day 2, those transduced with MigR1 encoding CD44ICD were well fused (bar represents 3 mm; ^*P < .001 versus MigR1; SD; n = 5). (B) Mouse bone marrow cells were transduced with the retroviral vector MigR1 empty or encoding CD44FL, CD44ΔE, or CD44ICD, treated with M-CSF supplemented with RANKL for only 2 days to differentiate into osteoclasts, then reacted for TRAP, an osteoclast marker. Whereas very few macrophages transduced with empty vector or vector encoding CD44FL or CD44ΔE were fused by day 2, macrophages transduced with CD44ICD were highly differentiated into TRAP⁺ multinucleated osteoclasts (bar is 2 mm; ^*P < .001 versus MigR1; SD; n = 6). (C) CD44ICD promotes the fusion of peritoneal macrophages but does not induce the fusion of 3T3 cells. Rat peritoneal macrophages and 3T3 cells were cultured in fusogenic conditions for 18 hours and 48 hours, respectively, following transduction with MigR1 either empty or encoding CD44ICD. CD44ICD strongly promoted the fusion of rat peritoneal macrophages (bar represents 1 mm; ^*P < .001 versus MigR1; SD; n = 4).