Catabolism of ¹²⁵I-labeled anti-mIgM μ chain is diminished in transfectants with altered γ chain, but it is increased by BCR ligation.

(A) FcαR on IIA1.6 cells expressing FcαR and either WT γ chain (wt), γ chain with IIA ITAM (IIA ITAM), or γ chain with Y → F mutation (Y → F) were ligated at 4°C with the anti-FcαR mAb My43 followed by ¹²⁵I-labeled antimouse IgM μ chain. The cells were then incubated at 37°C without further treatment or with additional cross-linking of BCR with antimouse IgG. After an 8-hour incubation at 37°C, supernates were harvested, and TCA-soluble supernate counts were measured as described in “Materials and methods.” The solid bars represent the amount of TCA-soluble counts released in the absence of BCR cross-linking. The hatched bars represent the amount of TCA-soluble counts released when BCR was cross-linked. Error bars indicate range of duplicate samples. The results are representative of 3 experiments. (B) IgA catabolism is defective in altered γ-chain transfectants but is restored by BCR ligation. FcαR transfectants with WT γ chain (WT, lane 1), γ chain with IIA ITAM (IIA, lane 2), or γ chain with Y → F mutation (Y → F, lane 3) were incubated with 50 μg/mL human myeloma IgA. After 4 hours the cells were washed and lysed, and intracellular IgA and IgA catabolism products were detected by SDS-PAGE and anti-IgA Western blot. Addition of 10 μg/mL anti-MIgG to the incubation mixture did not affect IgA catabolism by the WT γ-chain transfectant (lane 2), but the addition altered IgA catabolism by the IIA ITAM (lane 4) and Y → F (lane 6) γ-chain transfectants. Three separate experiments yielded the same result.