Sera were diluted in PBS starting at 1/10 and incubated for 2 h at 37°C. After washing, a mixture of mouse mAb anti-rat κ and anti-λ chain-specific peroxydase-conjugated Ab (clone MARK-1/MARL-15, Abd Serotec) were added at a dilution of 1:2000 in PBS for 1 h. Purified rat mAb IgM, IgG, IgA and IgE isotypes (Abd Serotec, Jackson ImmunoResearch, BD Biosciences) were added at different known concentrations and used as standard.

After three washes, TMB substrate reagent (Becton Dickinson) was added and the reaction was stopped after 10 min by the addition of 2 N H2SO4. Absorbance was read at 490 nm. Serum Ig concentrations were determined by extrapolating absorbance values of sera dilutions in the linear range of the dilution curves to those of isotype Smad inhibitor standard controls. Protein concentration of serum was measured (BCA Protein Assay Kit, Pierce, Rockford, IL, USA).

Cell Cycle inhibitor A standard curve dilution of monoclonal purified rat IgM and dilutions of rat serum (1/10 and 1/100) (20 μL/line) were electrophoresed in 12% SDS-polyacrylamide gels. After semi-dry transfer, the nitrocellulose membranes were blocked in 5% dry milk in PBS with 0.05% Tween-20 for 1–2 h and incubated overnight at 4°C with Peroxydase-conjugated goat anti-rat IgM μ-specific Ab (from Jackson Laboratories) at 1 μg/mL. The binding was visualized with enhanced chemiluminescence and quantified using a Fuji LAS 4000 (Fujifilm) imaging system and Multi Gauge V3.0 software GABA Receptor (Fujifilm). IgM KO rats (haplotype RT1u for MHC I and II) were immunized against donor LEW.1A alloantigens (haplotype RT1a for MHC I and II) by three successive skin grafts separated by 1 wk each and grafted with a LEW.1A heart 1 wk after the last skin transplant. Heart transplantation was performed heterotopically in the abdomen. Heart allograft survival was evaluated through abdominal palpation and rejection was defined as total cessation of beating and confirmed by visual inspection after laparotomy 32, 33. Anti-donor Ab were analyzed in the sera of WT or IgM KO rats harvested at day 0 before skin transplantation, at days 20 and 30 after skin transplantation and at rejection

or later time points if hearts were not rejected. Heat-inactivated sera (dilutions 1/10, 1/100 and 1/1000 and 1/5000) were incubated with donor T cells, washed and alloAb bound were detected using rat anti-IgG or IgM-specific Ab. As negative control, sera were incubated with recipient T cells. Results were reported as the mean channel fluorescence using donor T cells – mean channel fluorescence using donor T cells±SD. Results are presented as the means±SD. Statistical analyses were performed by a Mann–Whitney test for laboratory analyses and Kaplan–Meier log rank test for graft survival using GraphPad Prism 4 software (GraphPad Software, La Jolla, CA, USA). Differences associated with probability values of p<0.05 were considered statistically significant.