6 – Antigens and antigen processing

Since the first crystal structure determinations of alphabeta T cell receptors (TCRs) bound to class I MHC-peptide (pMHC) antigens in 1996, a sizable database of 24 class I and class II TCR/pMHC complexes has been accumulated that now defines a substantial degree of structural variability in TCR/pMHC recognition. Recent determination of free and bound gammadelta TCR structures has enabled comparisons of the modes of antigen recognition by alphabeta and gammadelta T cells and antibodies. Crystal structures of TCR accessory (CD4, CD8) and coreceptor molecules (CD3epsilondelta, CD3epsilongamma) have further advanced our structural understanding of most of the components that constitute the TCR signaling complex. Despite all these efforts, the structural basis for MHC restriction and signaling remains elusive as no structural features that define a common binding mode or signaling mechanism have yet been gleaned from the current set of TCR/pMHC complexes. Notwithstanding, the impressive array of self, foreign (microbial), and autoimmune TCR complexes have uncovered the diverse ways in which antigens can be specifically recognized by TCRs.

How TCRs Bind MHCs, Peptides, and Coreceptors

It was originally thought that a cell's major histocompatibility complex (MHC) class I molecules presented peptides derived exclusively from proteins synthesized by the cell itself. However, in some circumstances, antigens from the extracellular environment can be presented on MHC class I molecules and stimulate CD8(+) T-cell immunity, a process termed cross-presentation. Cross-presentation was originally discovered as an obscure phenomenon in transplantation immunity. However, it is now clear that it is a major mechanism by which the immune system monitors tissues and phagocytes for the presence of foreign antigen. Cross-presentation is the only pathway by which the immune system can detect and respond to viral infections or mutations that exclusively occur in parenchymal cells rather than in bone marrow-derived antigen-presenting cells (APCs). Professional APCs, such as dendritic cells, are the principal cells endowed with the capacity to cross-present antigens. In this process, the APCs acquire proteins from other tissue cells through endocytic mechanisms, especially phagocytosis or macropinocytosis. The internalized antigen can then be processed through at least two different mechanisms. In one pathway, the antigen is transferred from the phagosome into the cytosol, where it is hydrolyzed by proteasomes into oligopeptides that are then transported by the transporter associated with antigen processing to MHC class I molecules in the endoplasmic reticulum or phagosomes. In a second pathway, the antigen is cleaved into peptides by endosomal proteases, particularly cathepsin S, and bound by class I molecules probably in the endocytic compartment itself. Depending on the nature of the antigen, one or both of these pathways can contribute to cross-presentation in vivo. The outcome of cross-presentation can be either tolerance or immunity. Which of these outcomes occurs is thought to depend on whether antigens are acquired by themselves alone, leading to tolerance, or with immunostimulatory signals, leading to immunity. One source of such signals is from dying cells that release immunostimulatory 'danger' signals that promote the generation of immunity to their cellular antigens. In addition to the critical role of cross-presentation in normal immune physiology, this pathway has considerable potential for being exploited for developing subunit vaccines that elicit both CD4(+) and CD8(+) T-cell immunity.

Cross-presentation: underlying mechanisms and role in immune surveillance

Bacterial superantigens

Proteasome and peptidase function in MHC-class-I-mediated antigen presentation.

THE study of serological reactions has for years attracted many workers. There have been fierce controversies between the exponents of differing theories, and these controversies have stimulated both thought and work. During the last thirty years progress has been very rapid, and continues to be so, although relatively little work has been possible in Great Britain during the war years. The production of a revised edition of this extremely valuable book is therefore most welcome. The Specificity of Serological Reactions By Dr. Karl Landsteiner. Revised edition. With a Chapter on Molecular Structure and Intermolecular Forces, by Linus Pauling. Pp. xiv + 310. (Cambridge, Mass.: Harvard University Press; London: Oxford University Press, 1945.) 28s. net.

The Specificity of Serological Reactions

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6 – Antigens and antigen processing

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References

How TCRs Bind MHCs, Peptides, and Coreceptors

Cross-presentation: underlying mechanisms and role in immune surveillance

Bacterial superantigens

Proteasome and peptidase function in MHC-class-I-mediated antigen presentation.

The Specificity of Serological Reactions

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