Figure 2.

Chemical crosslinking of receptor-bound ¹²⁵I-TGF-β was performed to determine the presence or absence of the receptors on normal plasma cells (A) and PCT cells (B and C). SDS/PAGE of receptor immunoprecipitates of purified (A, lane 2) and unpurified (A, lane 1) plasma cells from an IL-6 transgenic mouse demonstrates the presence of TβRI and TβRII at levels comparable to the control B cell lymphoma P388 (A, lane 3). Loss of expression occurs in the in vitro PCT cell lines X24 and 1165 (B, lanes 2 and 3, respectively), again compared with pre-B lymphomas P388 and L1210 (B, lanes 4 and 5, respectively) and the M1 myeloid leukemic cell line (B, lane 1). Crosslinking of purified primary PCTs (C, lanes 2 and 3) and a transplanted PCT (TP) (C, lane 1) demonstrates that loss of receptor expression occurs in vivo, before cell culture adaptation; here compared with B cell lymphoma CH31 (C, lane 4). (+) and (-) indicate the addition of unlabeled TGF-β1 to compete for specific binding.