Injections of IgG-containing sera from WT-cGVHD recipients lead to tissue deposition of IgG and persistence of cGVHD in IgH^µγ1-transient-cGVHD recipients. BALB/c recipients were lethally irradiated (850 cGy, TBI) and given CD25-depleted splenocytes (75 × 10⁶) and TCD-BM (2.5 × 10⁶) cells from IgH^μγ1 DBA/2 donors to set up IgH^µγ1-transient-cGVHD recipients. Sera were harvested from WT-cGVHD or IgH^µγ1-no-cGVHD recipients at 30 to 45 days after HCT. Serum aliquots (250 µL) were injected IV into IgH^µγ1-transient-cGVHD recipients at 40, 45, 50, and 55 days after HCT. Recipients were monitored for cGVHD development for up to 60 days after HCT. N = 8 from 2 replicate experiments. (A) Clinical cutaneous GVHD scores. A representative photograph taken at day 60 is shown. (B) Histopathology of the GVHD target tissues liver, lung, salivary gland, and skin was evaluated. A representative photomicrograph (original magnification, ×200); means ± SE of histopathology scores from 6 recipients are shown. Arrows indicate infiltration in the liver, lymphocytic bronchiolitis, infiltration and loss of ductal structure in the salivary gland, hyperplasia in the epidermis, expansion of dermis and loss of subcutaneous fat. (C) Representative flow cytometry patterns of CD4⁺CD8⁺ double-positive thymocytes, and means ± SE of percentages and yields of double-positive thymocytes are shown. N = 4. (D) Yield of donor CD5.1⁺TCRβ⁺CD4⁺ T in the spleen and PLNs (means ± SE, N = 4). (E) Percentages of donor CD5.1⁺IFN-γ⁺CD4⁺ and CD5.1⁺IL-17⁺CD4⁺ T cells (means ± SE, N = 4-6). (F) Histoimmunofluorescent staining of mouse IgG (green) and DAPI (blue, nucleus) in thymus and skin sections. A representative photomicrograph (original magnification, ×100) and means ± SE (N = 4) of fluorescent intensity are shown (* P < .05, ** P < .01, ***P < .001).