Enhanced binding of low-affinity antibodies interacting simultaneously with targeted cell surface molecules and Fc receptor.

The possibility that different forms of class I molecules might be expressed on the cell surface of lymphocytes has been investigated periodically over the past several decades. A series of major histocompatibility complex (MHC) class I-specific monoclonal antibodies, including the commonly used antibodies 64-3-7 and 25-D1.16, bind B cells differentially, suggesting the existence of differentially expressed class I-associated cell surface determinants on B lymphocytes. However, the ability of antibodies to bind cells is determined by the sum of interactions between the antibodies and the molecules expressed on the cell surface. The interactions of class I-specific antibodies with B cells were dissected, revealing dual specificity of the antibodies for the targeted class I molecules, as well as to Fc receptors preferentially expressed by B cells. We demonstrate that antibodies simultaneously bind targeted class I molecules and Fc receptors expressed on the surface of B cells. Simultaneous binding to two cell surface structures significantly enhances the class I-specific binding pattern of certain antibodies by increasing their avidity, leading to apparent cell-specific differences in MHC expression patterns. We conclude that no differences in MHC structures need be postulated to account for the observed binding patterns.

T cell receptor interactions with class I heavy-chain influence T cell selection.

The interaction of the T cell receptor (TCR) with peptide in the binding site of the major histocompatibility complex molecule provides the basis for T cell recognition during immune surveillance, repertoire development, and tolerance. Little is known about the extent to which repertoire selection is influenced directly by variation of the structure of the class I heavy chain. We find that the 2C TCR, normally positively selected in the context of the K(b) molecule, is minimally selected into the CD8 lineage in the absence of antigen-processing genes. This finding underscores the importance of peptides in determining the positive-selecting class I ligands in the thymus. In contrast, K(bm3), a variant class I molecule that normally exerts a negative selection pressure on 2C-bearing T cells, positively selects 2C transgenic T cells into the CD8 lineage in an antigen-processing gene-deficient environment. These findings indicate that structural changes in the heavy chain can have direct influence in T cell recognition, from which we conclude that the nature of TCR interaction with class I heavy chain influences the array of TCRs selected during development of the functional adult repertoire.

Prevalent class I-restricted T-cell response to the Theiler's virus epitope Db:VP2121-130 in the absence of endogenous CD4 help, tumor necrosis factor alpha, gamma interferon, perforin, or costimulation through CD28.

C57BL/6 mice mount a cytotoxic T-lymphocyte (CTL) response against the Daniel's strain of Theiler's murine encephalomyelitis virus (TMEV) 7 days after infection and do not develop persistent infection or the demyelinating syndrome similar to multiple sclerosis seen in susceptible mice. The TMEV capsid peptide VP2121-130 sensitizes H-2Db+ target cells for killing by central-nervous-system-infiltrating lymphocytes (CNS-ILs) isolated from C57BL/6 mice infected intracranially. Db:VP2121-130 peptide tetramers were used to stain CD8(+) CNS-ILs, revealing that 50 to 63% of these cells bear receptors specific for VP2121-130 presented in the context of Db. No T cells bearing this specificity were found in the cervical lymph nodes or spleens of TMEV-infected mice. H-2(b) mice lacking CD4, class II, gamma interferon, or CD28 expression are susceptible to persistent virus infection but surprisingly still generate high frequencies of CD8(+), Db:VP2121-130-specific T cells. However, CD4-negative mice generate a lower frequency of Db:VP2121-130-specific T cells than do class II negative or normal H-2(b) animals. Resistant tumor necrosis factor alpha receptor I knockout mice also generate a high frequency of CD8(+) CNS-ILs specific for Db:VP2121-130. Furthermore, normally susceptible FVB mice that express a Db transgene generate Db:VP2121-130-specific CD8(+) CNS-ILs at a frequency similar to that of C57BL/6 mice. These results demonstrate that VP2121-130 presented in the context of Db is an immunodominant epitope in TMEV infection and that the frequency of the VP2121-130-specific CTLs appears to be independent of several key inflammatory mediators and genetic background but is regulated in part by the expression of CD4.

A region of conformational variability outside the peptide-binding site of a class I MHC molecule.

Peptide binding is known to influence the conformation of the surface of class I molecules as detected with mAbs and TCR. A new conformationally sensitive epitope on the mouse class I molecule Kb is defined by mAb AF6-88.5. The recognized structure is affected by amino acid substitutions in any of the three external domains of the class I heavy chain and, in addition, is influenced by the substitution of human for mouse beta2-microglobulin. Interestingly, the epitope for this Ab is not affected by mutations within the peptide-binding cleft or by the nature of the peptide bound. These findings indicate that the effect of a change in one domain of class I can radiate to other parts of the molecule. Furthermore, the existence of conformationally sensitive structures outside of the peptide-binding site suggests the possibility that class I molecules may change their structure in response to binding by receptors and ligands such as the TCR and the coligand CD8. Such structural changes may represent signals that can influence cellular activation events.