The continuous antigenic regions in the second domain of the beta chain of human MHC DR2 antigen: antigenic profile of the entire extracellular part of the chain

A Site for CD4 Binding in the β1 Domain of the MHC Class II Protein HLA-DR1

Using a lymphocyte binding assay, we have previously demonstrated that the CD4 protein can mediate cell adhesion by direct interaction with MHC class II molecules. In this report, we have used this assay to test whether synthetic peptides, corresponding to DRβ sequences, could inhibit CD4-class II adhesion. A peptide derived from sequences within the β1 domain (DRβ41–55), as well as two peptides derived from sequences within the β2 domain (DRβ121–135 and DRβ141–155), were shown to inhibit CD4-class II adhesion. Inasmuch as a site for CD4 binding in the β2 domain had been previously documented, these studies were designed to investigate the role of the β1 domain as an additional site of interaction with CD4. Sixteen site-specific mutations were engineered within the β1 domain of DRβ1*0101. Several mutations were shown to disrupt CD4-dependent T cell activation. Based on these results, we propose a model for the molecular interaction of CD4 with MHC class II proteins in which both the β1 and β2 domains of class II interact with the two amino-terminal Ig-like domains of CD4.

Molecular mimicry in the pathogenesis of AIDS: the HIV/MHC/mycoplasma triangle

The immune defects characterizing infection with the human immunodeficiency virus (HIV) and culminating in the acquired immune deficiency syndrome (AIDS) are the result of a multifactorial disease process, components of which are the occurrence of autoimmune phenomena and opportunistic infection. In this discussion, the observation that both the HIV-1 gp 120 envelope and Mycoplasma genitalium adhesin proteins share an area of significant similarity with the CD4-binding site of the class II major histocompatibility complex (MHC) proteins is placed in this perspective and mechanisms by which interaction within this triad could contribute to the T-cell dysfunction, T-cell depletion, Th1-cell-->Th2-cell shift, B-cell proliferation, hyperglobulinemia and antigen-presenting cell dysfunction observed during the development of AIDS are proposed.

Interactions of CD4 with MHC class II molecules, T cell receptors and p56lck

CD4 and CD8 are members of the immunoglobulin supergene family of proteins, and function as co-receptors with the T cell receptor (TCR) in binding MHC class II or class I molecules, respectively. Within this multimeric complex, CD4 interacts with three distinct ligands. CD4 interacts through its D1 and D2 domains with MHC class II proteins, through its D3 and D4 domains with T cell receptors, and through its cytoplasmic tail with p56lck, a src-related, protein tyrosine kinase. Each of these interactions is important in the function of CD4 and will be discussed in turn.

HIV envelope protein is recognized as an alloantigen by human DR-specific alloreactive T cells

Recent studies from this laboratory reported the mapping of the full profile of T-cell allorecognition regions of HLA-DR2 beta subunit. The results indicated the presence of an allodeterminant within DR2 beta regions 141-156. In another study, we have shown that this allodeterminant is one of five regions of structural homology between the DR2 beta molecule and the HIV-envelope protein gp120 region 254-268. The fact that gp120 peptide 254-268 is homologous to the allodeterminant within the DR2 beta region 141-156 prompted us to investigate whether synthetic gp120 peptide 254-272 is recognized by human DR2-specific alloreactive T-cell lines. Five human alloreactive T-cell lines were prepared that were specific for the DR2 molecule and did not recognize DR1. These lines mounted in vitro proliferative responses to the allodeterminant peptide DR141-156 and also responded to the DR-similar peptide gp254-272. Removal of the residues 262-272 from the gp120 peptide (i.e., peptide 254-263) resulted in essentially complete loss of its proliferative activity. The effect of deletion of three residues of homology (Val-Val-Ser) at the N terminal (i.e., peptides DR145-156 and gp257-272) was examined. Peptide DR2 beta 145-156 exhibited very low stimulating activity, whereas peptide gp 257-272 did not cause T-cell proliferative responses in any of the alloreactive T-cell lines. The T-cell lines did not respond to unrelated peptide controls, thus further confirming the specificity of these responses. These findings indicate that the virus is recognized as an alloantigen by human alloreactive DR2-specific T cells.

HLA-DR peptide inhibits HIV-induced syncytia

The infectivity of the human immunodeficiency virus (HIV) is related to the structure of its envelope protein, gp160, which is responsible for viral entry. We considered the possibility that a structural homology between gp160 and major histocompatibility complex (MHC) molecules might be associated with the extraordinary affinity that gp120 has for its receptor, CD4. Amino acid sequence comparisons revealed five regions of structural similarity between the HLA-DR beta molecule and gp160. The DR2 beta synthetic peptides containing these regions were examined for their ability to block HIV-induced syncytia formation using a 51Cr release assay. The peptide beta 141-155 inhibited the formation of syncytia whereas the other four DR beta peptides with gp160 similarity did not. Our results indicate that this region in gp120, which is similar to an HLA-DR region, is crucial to T cell-gp120 interactions, and should be considered in the design of future vaccines.

Mapping of the full profile of T cell allorecognition regions on HLA-DR2 beta subunit

Alloreactive human T cell lines were developed by repeated in vitro restimulation with alternate allogeneic cells of similar DR2 serotype, but differing at other HLA loci. These polyclonal T cells recognized DR specificities in common with all DR2 serotypes. The allorecognition profiles of DR2 beta by the T cell lines were mapped by consecutive uniform-sized overlapping peptides encompassing the entire extracellular and intracellular regions of the DR beta subunit. It was found that the T cell allorecognition sites are focused around the first and third polymorphic regions of the N-terminal domain, and interestingly, include conserved areas in the DR beta second domain as well as the intracellular segment beta 222-237. These findings have important implications for allorecognition. Comparison with the previous analysis of antibody-binding regions of DR2 beta revealed some similarities and differences in the antibody and T cell alloresponses to this histocompatibility antigen.

DR beta peptides block the antigen-specific response but not the alloresponse of a dual-reactive T-cell clone

It is generally assumed that alloreactivity and the response to foreign antigens are equivalent T-cell recognition events. We have addressed this issue by examining the antigen presentation of beef insulin by two DR1-restricted human T-clones. One of the clones was dual-reactive and exhibited alloreactivity. By employing 20 synthetic consecutive overlapping peptides representing the entire extracellular (residues 1-198) and intracellular (residues 222-273) parts of the DR2 beta molecule, the effects of each of these peptides on antigen presentation and alloreactivity were examined. The DR1-restricted response to beef insulin by both clones was inhibited by peptides from DR2 beta region 1-25. Other DR2 beta (residues 21-198 en 222-237) or control peptides had no effect. The DR3-associated alloresponse of the dual-reactive clone was not inhibited by any peptide. These observations suggest that recognition of antigen is fundamentally different from allorecognition.

Alloreactive T cell recognition of the HLA-DR beta N-terminal polymorphic region

T cell alloreactivity, originally discovered as a tissue transplantation effect, is believed to be a manifestation of the normal major histocompatibility complex (MHC) restriction of antigen presentation by accessory cells to T cells. The molecular features of the human class II-MHC proteins (HLA) which are recognized by alloreactive T cells are not at present understood, although they are clearly related to the polymorphic nature of the MHC proteins. Human CD4+ T cell clones were selected by response to the HLA-DR2 peptides beta 1-15 or beta 51-65, in an MHC-restricted manner. In addition, these clones respond to cell lines expressing the DR2 haplotype, without the requirement for accessory antigen presenting cells. DR2 beta peptide 1-15 blocks the T cell alloresponse and polymorphic residues are shown to stimulate the peptide-specific response of these clones. Thus, the polymorphic residues contained within the DR beta sequence 1-15 are demonstrated to be directly recognized by alloreactive T cells.