The effect of cyclophosphamide on the resident macrophages in thymus (rat)

Stain: Immunoperoxidase staining with diaminobenzidin (DAB) and hematoxylin counterstained on frozen section. A single injection with cyclophosphamide (CP, 70 mg/ml) induces a transient cortical involution after 4 days. The darkly stained cortex of th normal thymus (A1, A2) decreases its dark staining completely (B1, B2), due to loss of cortical thymocytes. The thymus recovers to normal about 10 days later (C). The monoclonal antibody ED-1 stains the rat homologue of human CD68, a single chain glycoprotein of 110kDa that is expressed predominantly on the lysosomal membrane of tissue macrophages (and of myeloid cells). A1, A2: Normal thymus showing most resident macrophages localised in the medulla (A2) (A1-1; A2), dark brown cytoplasmic stained macrophages in a lightly stained medulla. The darkly stained cortex (A1-2) contains much less resident macrophages; B1, B2: Four days after CP treatment the macrophages have occupied equally the depleted cortex and the darker stained medulla; C: Ten days later the status has been almost recovered to normal distribution of macrophages and lymphocytes; (3) shows septa or trabeculae dividing the thymus in lobes.