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Previous studies from our laboratory showed that B 10.A mice are high responders to pigeon cytochrome c fragment 81-104, whereas'B 10.A(5R) mice are low responders. In the present studies, the C-terminal cyanogen bromide cleavage fragment and homologous synthetic peptides of tobacco horn worm moth cytochrome c were shown to be immunogenic in both B10.A and B10.A(5R) mice. These strains, however, showed different patterns of cross-reactivity when immune lymph node T cells were stimulated with cytochrome c fragments from other species. To examine the two patterns of responsiveness at a clonal level, cytochrome c fragment-specific T cell hybridomas were made and found to secrete interleukin 2 in response to antigen. The patterns of cross- reactivity of these B 10.A and B 10.A(5R) clones were similar to that seen in the whole lymph node population. Surprisingly, when these clones were tested for major histocompatibility complex (MHC)-restricted antigen recognition, they were all found to respond to antigen with both B10.A and B10.A(5R) antigen-presenting cells (APC). Furthermore, the cross-reactivity pattern appeared to be largely determined by the genotype of the APC, not the genotype of the T cell clone. That is, a given T cell clone displayed a different fine specificity when assayed with B10.A or B10.A(5R) APC. This observation indicates that the APC MHC gene product and antigen interact during the stimulation of the T cell response and that as a consequence the specificity of antigen-induced T cell activation is influenced by these MHC gene products. (During the preparation of this manuscript it has come to our attention that results similar to our own, concerning the fine specificity of cytotoxic T cell clones, have been obtained by Dr. T. R. Hunig and Dr. M. J. Bevan, Massachusetts Institute of Technology, Boston, MA. T. R. Hunig and M. J. Bevan. 1981. Specificity of T-cell clones illustrates altered self hypothesis. Nature. 294:460.)

To test whether reduced hepatic uroporphyrinogen decarboxylase activity is a specific and intrinsic defect in porphyria cutanea tarda, we measured enzymatic activity in the livers of 17 patients with porphyria cutanea tarda, 12 "normal" control patients without liver disease, and 41 patients with other forms of porphyria, alcoholic liver disease, hemochromatosis, or chronic hepatitis. Enzyme activity in all the patients with porphyria cutanea tarda was lower than in the patients without this disease, except for one patient with alcohol-induced fatty liver. Reduction of hepatic iron stores by phlebotomy did not alter the enzymatic activity in porphyria cutanea tarda. We conclude that reduced hepatic uroporphyrinogen decarboxylase activity is a specific and intrinsic hepatic defect in porphyria cutanea tarda, but modulation of uroporphyrinogen synthesis by extrinsic factors is required for the full biochemical expression of the disease.

Treatment of human erythrocytes with tetrathionate or diamide resulted in extensive crosslinking of membranous and cytoskeletal proteins. Such treatment was followed by an incubation with phosphatidylcholine specific exchange protein to investigate the rate and extent of exchange of phosphatidylcholine between the erythrocytes and 14C-labeled phosphatidylcholine containing microsomal membranes or vesicles. Exchange profiles showed that the exchange of phosphatidylcholine is facilitated in treated cells when compared to control erythrocytes and, more importantly, that all of the phosphatidylcholine is exchangeable after protein crosslinking whereas in control cells only the phosphatidylcholine pool located in the outer layer of the membrane is exchangeable. These observations demonstrate that crosslinking of cytoskeletal and membraneous proteins enhances the rate of transbilayer movement of phosphatidylcholine considerably.