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As perhaps the staunchest advocates of repertoire purging as the central mechanism of immunological tolerance, we note with satisfaction a spate of recent, elegant papers which suggest an intrathymic clonal abortion model as the explanation for at least some examples of T-cell tolerance. This view agrees with the classical formulation of the Billingham-Brent-Medawar school of tolerance as a specific, central failure of immune responsiveness. Repertoire purging within the B-lymphocyte compartment remains much more controversial. There is no doubt that experimental models exist where the B cell is the reversible target of tolerance induction. The question is, in view of the ease of inducing autoantibody formation both in vivo and in vitro, just how relevant are such clonal anergy mechanisms to authentic self-tolerance? Arguments are presented that there must be two windows of tolerance susceptibility in the ontogeny of the B cell; one while it is maturing in the bone marrow, to prevent autoreactivity of high affinity to important accessible self-antigens; and a second soon after activation of pre-memory cells by exogenous antigen, to prevent fortuitous mutations towards high-affinity anti-self-reactivity establishing a forbidden clone. [ABSTRACT FROM AUTHOR]

Natural immunity to synthetic peptides (SP) derived from the sequences of a 3800 MW streptococcal antigen (SA) was found in human subjects. Significant serum IgG antibodies were detected both to the native SA and to peptides consisting of residues 3–13, l–15 and 1–20. Inhibition studies confirmed cross-reactivity between the native SA and SP. A series of short peptides with deletions at the amino and carboxy termini were then tested to determine the sequence of B-cell epitopes. Residues 8–13 and 1–6 bound significant serum IgG antibodies, but residues 8–13 were more effective and consistent in inhibiting human antibodies than residues 1–6. These results suggest that residues 8–13 constitute a major B-cell epitope but that residues 1–6 may represent a minor B-cell epitope. The human CD4 subset of T cells was then examined by stimulating the ceils with SA or SP and measuring the uptake of [3H] thymidinc ([3H]Td R). The cells were found to be sensitized in vivo to both the native SA and the SP and cross-reactivity between the SA and SP was shown by enrichment and depletion experiments on antigen-coated monocytes. As with the B-cell epitope, the series of short peptides was used to stimulate CD4 cells, in order to determine the T-cell epitope. Residues 6–15 were the shortest SP which stimulated significant [3H]Td R uptake and this peptide was designated as a T-cell epitope. The results suggest that natural oral immunization with Streptococcus mutans induces serum antibodies and T-cell sensitization to a peptide in which a T-cell epitope (residues 6–15) overlaps with a B-cell epitope (residues 8–13). Furthermore, a comparison between linear and cycled peptides suggests that unlike immunogenicity which is commonly enhanced by the more rigid cyclized peptides, antigenicity is favoured by linear peptides. This was evident not only for antibodies but also for T-cell proliferative responses. [ABSTRACT FROM AUTHOR]

Different T-cell subsets participating in immune responses were detected at a high frequency by a modified virus plaque assay (VPA). By using the modified VPA, different activated T-cell subsets generated in primary immune responses, helper and suppressor T cells participating in antibody formation, and effector T cells involved in the delayed-type hypersensitivity (DTH) reaction were enumerated directly without in vitro antigen stimulation. The frequency of detection in immune systems used was 7·5–17·7 V-PFC/103 spleen cells. Although neither helper T cells for antibody formation nor effector T cells for DTH reaction were detected as V-PFC at a high frequency by the original VPA, it was also found in secondary immune response that Lyt 1 positive, antigen-specific helper and effector T-cell subsets, and cyclophosphamide (CY)-resistant precursors were enumerated at a high frequency by the modified VPA when they received in vitro antigen stimulation, and that the proliferative stage of these cells was critical for the development of V-PFC. [ABSTRACT FROM AUTHOR]

Unlike the suppression of a primary response to a thymus-dependent antigen, pretreatment of mice with anti-IgD serum has no effect on the response to a thymus-independent (TI) antigen. It has also been shown that anti-IgD serum injected parenterally into mice 2 days previously, has no suppressive effect on non-specific responsiveness to an intraperitoneal injection of LPS. There is a similar lack of effect on the non-specific element of responsiveness to a TI antigen. These results provide in vivo confirmation of the results of previous in vitro experimentation. [ABSTRACT FROM AUTHOR]

The H-Y-expressing murine tumour, ET-5, specifically immunizes B6 female mice that have rejected it against H-Y-positive male skin grafts, yet fails to prime their spleen cells for the generation of H-Yspecific cytotoxic T lymphocytes (CTL). ET-5 also fails to prime relevant congenic hosts to generate CTL specific for H-3 or H-25 minor H antigens, or major histocompatibility complex (MHC) class I antigens, all of which are expressed in immunogenic form by ET-5. Nonetheless, C3H.SW mice, which are MHC-compatible with B6 mice in which ET-5 originated, but differ from B6 at many minor H loci, can be primed to generate CTL directed against one or more unidentified minor H antigens. These CTL are conventional MHC-restricted, CD8+ T cells, and require priming in vivo for their generation. Significantly, C3H.SW females can be primed by ET-5 to generate B6-specific CTL, but not H-Y-specific CTL. Thus CTL priming is selective in the sense that ET-5 primes CTL specific for some of the antigens it expresses but not others. The basis for this selectivity is not known but, in the case of H-Y antigen, it appears not to result from an inability of ET-5 to express either H-2Dbencoded antigens, the restriction element for H-Y-specific CTL in B6 females, or H-Y itself in vivo. [ABSTRACT FROM AUTHOR]