Influence of the tapasin C terminus on the assembly of MHC class I allotypes

Several endoplasmic reticulum proteins, including tapasin, play an important role in major histocompatibility complex (MHC) class I assembly. In this study, we assessed the influence of the tapasin cytoplasmic tail on three mouse MHC class I allotypes (H2-K(b), -K(d), and -L(d)) and demonstrated that the expression of truncated mouse tapasin in mouse cells resulted in very low K(b), K(d), and L(d) surface expression. The surface expression of K(d) also could not be rescued by human soluble tapasin, suggesting that the surface expression phenotype of the mouse MHC class I molecules in the presence of soluble tapasin was not due to mouse/human differences in tapasin. Notably, soluble mouse tapasin was able to partially rescue HLA-B8 surface expression on human 721.220 cells. Thus, the cytoplasmic tail of tapasin (either mouse or human) has a stronger impact on the surface expression of murine MHC class I molecules on mouse cells than on the expression of HLA-B8 on human cells. A K408W mutation in the mouse tapasin transmembrane/cytoplasmic domain disrupted K(d) folding and release from tapasin, but not interaction with transporter associated with antigen processing (TAP), indicating that the mechanism whereby the tapasin transmembrane/cytoplasmic domain facilitates MHC class I assembly is not limited to TAP stabilization. Our findings indicate that the C terminus of mouse tapasin plays a vital role in enabling murine MHC class I molecules to be expressed at the surface of mouse cells.

Amyloid precursor-like protein 2 increases the endocytosis, instability, and turnover of the H2-K(d) MHC class I molecule

The defense against the invasion of viruses and tumors relies on the presentation of viral and tumor-derived peptides to CTL by cell surface MHC class I molecules. Previously, we showed that the ubiquitously expressed protein amyloid precursor-like protein 2 (APLP2) associates with the folded form of the MHC class I molecule K(d). In the current study, APLP2 was found to associate with folded K(d) molecules following their endocytosis and to increase the amount of endocytosed K(d). In addition, increased expression of APLP2 was shown to decrease K(d) surface expression and thermostability. Correspondingly, K(d) thermostability and surface expression were increased by down-regulation of APLP2 expression. Overall, these data suggest that APLP2 modulates the stability and endocytosis of K(d) molecules.

Similarities and differences in tapasin interaction with disparate murine MHC class I allotypes (93.11)

Tapasin is a key molecule in the major histocompatibility complex (MHC) class I peptide-loading complex. In this study, we first assessed the influence of the tapasin C-terminus on H2-Kb, -Kd, and -Ld in mouse cells. Truncated mouse tapasin (lacking the transmembrane and C-terminal region) was unable to associate with TAP or any of three murine MHC class I allotypes, and did not assist murine MHC class I folding and surface expression. A tapasin mutant with a single substitution in the C-terminus (K408W) increased the amount of Kd (but not Ld) in the open, peptide-free form, relative to wild type tapasin. We investigated whether Kd was also particularly responsive to other tapasin mutations, and found that a C95S substitution in tapasin also caused an elevation in open Kd (but not open Ld or Kb). Folded Kd was found to be amply present in association with wild type tapasin, and even more so with tapasin K408W and C95S. In summary, our results suggest that the C-terminus of mouse tapasin plays a vital role in the surface expression of murine MHC class I molecules, and that mouse MHC allotypes vary in their sensitivity to specific alterations in tapasin sequence and in their release from tapasin after folding. Overall, our results reveal new complexity in tapasin’s interactions with polymorphic MHC class I molecules. (NIH Grant GM57428)

Regulation of Antigen Presentation by Amyloid Precursor-Like Protein 2 (36.1)

The amyloid precursor-like protein 2 (APLP2) is a secreted protein that is ubiquitously expressed. We have found that APLP2 over-expression decreases the quantity of H2-Kd molecules on the plasma membrane. In addition, Kd thermostability was improved by APLP2 down-regulation, suggesting that APLP2 affects Kd peptide selection and/or retention. The effects of APLP2 on Kd are likely related to their physical association, as pulse-chase experiments demonstrated interaction for up to two hours after Kd synthesis. Furthermore, significant co-localization of Kd molecules with APLP2 was observed in vesicular structures both at steady state and during endocytosis of Kd molecules from the cell surface. Mutagenesis studies revealed that APLP2/MHC association is influenced by sequences in the peptide-binding region and the α3 domain of the MHC class I heavy chain. β2m has an impact on APLP2/MHC interaction, since introduction of human β2m into mouse fibroblasts increased the level of interaction between H2-Dd and APLP2. In summary, APLP2 affects MHC class I cell surface expression and thermostability by an interaction controlled by more than one domain of the MHC class I heavy chain and by β2m. These data support a model in which APLP2 regulates MHC class I trafficking, and thereby antigen presentation. (NIH Grants AI54645, GM57428, and GM74876)

Effect of linkage of transduction domain sequences to a lymphoma idiotype DNA vaccine on vaccine effectiveness

Patient idiotype-specific vaccines for treatment of non-Hodgkin's lymphoma have shown promise in clinical trials, encouraging efforts to enhance the effectiveness of idiotype vaccines further. It has previously been found that for some other types of experimental vaccines, the addition of transduction domains has improved vaccine immunogenicity. Transduction domains are short amino acid sequences that are capable of increasing transport through cellular membranes. In this study, we tested murine B cell 38C13 lymphoma idiotype DNA vaccines with human immunodeficiency virus (HIV) Tat-derived transduction sequences for efficacy against 38C13 challenge. The rate of tumor onset was similar for the idiotype and transduction domain-conjugated idiotype vaccine groups. At days 22-23 postchallenge, the number of surviving mice was significantly higher in the group that had received a DNA vaccine consisting of the 38C13 idiotype sequence plus modified Tat transduction sequence, in comparison with the group that received idiotype-only vaccines. Although the overall survival difference was not statistically significant following day 24, a trend toward an increased survival rate for mice receiving idiotype plus Tat-derived transduction domains was maintained through day 106 postchallenge. Thus, the addition to idiotype vaccines of specific sequences that facilitate intracellular transport may have potential to improve the effectiveness of such vaccines.

A Charged Amino Acid Residue in the Transmembrane/Cytoplasmic Region of Tapasin Influences MHC Class I Assembly and Maturation

Tapasin influences the quantity and quality of MHC/peptide complexes at the cell surface; however, little is understood about the structural features that underlie its effects. Because tapasin, MHC class I, and TAP are transmembrane proteins, the tapasin transmembrane/cytoplasmic region has the potential to affect interactions at the endoplasmic reticulum membrane. In this study, we have assessed the influence of a conserved lysine at position 408, which lies in the tapasin transmembrane/cytoplasmic domain. We found that substitutions at position K408 in tapasin affected the expression of MHC class I molecules at the cell surface, and down-regulated tapasin stabilization of TAP. In addition to affecting TAP interaction with tapasin, the substitution of alanine, but not tryptophan, for the lysine at tapasin position 408 increased the amount of tapasin found in association with the open, peptide-free form of the HLA-B8 H chain. Tapasin K408A was also associated with more folded, beta(2)-microglobulin-assembled HLA-B8 molecules than wild-type tapasin. Consistent with our observation of a large pool of tapasin K408A-associated HLA-B8 molecules, the rate at which HLA-B8 migrated from the endoplasmic reticulum was slower in tapasin K408A-expressing cells than in wild-type tapasin-expressing cells. Thus, the alanine substitution at position 408 in tapasin may interfere with the stable acquisition by MHC class I molecules of peptides that are sufficiently optimal to allow MHC class I release from tapasin.

Tapasin Decreases Immune Responsiveness to a Model Tumor Antigen

The T-cell response against cancer is dependent on the cell surface presentation of tumor-associated or tumor-specific peptides by major histocompatibility complex (MHC) class I molecules. We found that tapasin, a chaperone protein that normally assists in the assembly of MHC class I molecules, is undetectable in an unstimulated pancreatic tumor cell line, Panc02, and only very weakly expressed after gamma-interferon stimulation. Transfection of tapasin into the Panc02 cells did not quantitatively increase MHC class I surface expression or detectably affect MHC class I association with peptide and beta(2)-microglubulin (beta(2)m). However, we found that transfected tapasin downregulated immune reactivity against a model tumor antigen, MUC1. Although tapasin has been previously shown by others to increase immune recognition of particular antigens, our results suggest that tapasin has a negative impact on the presentation of an immunodominant epitope from a specific model tumor antigen.

The Ig-Like Domain of Tapasin Influences Intermolecular Interactions

Presentation of antigenic peptides to T lymphocytes by MHC class I molecules is regulated by events involving multiple endoplasmic reticulum proteins, including tapasin. By studying the effects of substitutions in the tapasin Ig-like domain, we demonstrated that H-2L(d)/tapasin association can be segregated from reconstitution of folded L(d) surface expression. This finding suggests that peptide acquisition by L(d) is influenced by tapasin functions that are independent of L(d) binding. We also found that the presence of a nine-amino acid region in the Ig-like domain of mouse or human tapasin is required for association with L(d), and certain point substitutions in this sequence abrogate human, but not mouse, tapasin association with L(d). These data are consistent with a higher overall affinity between L(d) and mouse tapasin compared with human tapasin. In addition, we found that other point mutations in the same region of the tapasin Ig-like domain affect MHC class I surface expression and Ag presentation. Finally, we showed that the cysteine residues in the Ig-like domain of tapasin influence tapasin's stability, its interaction with the MHC class I H chain, and its stabilization of TAP. Mutagenesis of these cysteines decreases tapasin's electrophoretic mobility, suggesting that these residues form an intramolecular disulfide bond. Taken together, these results reveal a critical role for the tapasin Ig-like domain in tapasin function.

The amyloid precursor-like protein 2 and the adenoviral E3/19K protein both bind to a conformational site on H-2Kd and regulate H-2Kd expression

A protein of unknown physiological function, called amyloid precursor-like protein 2 (APLP2), forms an association with the murine class I molecule K(d) that is up-regulated by the presence of the adenoviral protein E3/19K. We have extended these findings to show that APLP2 and E3/19K associate preferentially with folded K(d) and not with the open form. APLP2 was detectable at the cell surface, but its surface expression was not up-regulated by the concurrent expression of K(d). Experimental down-regulation of APLP2 expression caused a consistent increase in the surface expression of K(d), indicating that APLP2 normally reduces K(d) surface expression. These data suggest a role for APLP2 in controlling the maturation of major histocompatibility complex class I molecules.

The interface between tapasin and MHC class I: identification of amino acid residues in both proteins that influence their interaction

Prior to the binding of antigenic peptide, a complex of chaperone proteins associates with the Major Histocompatibility Complex (MHC) class I heavy chain/beta2m heterodimer. Although each domain of the MHC class I heavy chain contains amino acid residues that influence chaperone binding, there are several pieces of evidence that point to an interaction between the MHC class I alpha/2/alpha3 domains and tapasin. In regard to the site on tapasin involved in the tapasin/MHC interface, we have found that a particular region of tapasin (containing amino acid residues 334-342) is necessary for the binding of tapasin to the MHC class I heavy chain. Our results also indicate that amino acids in this region of tapasin also affect the proportion of MHC class I open forms expressed at the cell surface and MHC class I egress from the endoplasmic reticulum. Based on these results and those obtained by other laboratories, a model for MHC class I/tapasin interaction is proposed.

HLA class I polymorphism has a dual impact on ligand binding and chaperone interaction

This article will describe coordinated analyses of how amino acid substitutions in the HLA class I antigen binding groove modify chaperone interaction and peptide ligand presentation. By parallel testing of ligand presentation and chaperone interaction with a series of natural HLA-B subtypes, this study has discovered that position 116 of the HLA-B15 class I heavy chain is pivotal in both peptide selection and control of interaction between the assembly complex and the class I heavy chain. Correlated with these qualitative differences in peptide selection and chaperone association are quantitative differences in the expression levels of the HLA molecules at the cell surface. These parallel studies, therefore, demonstrate that particular HLA class I polymorphisms can simultaneously influence ligand presentation and interaction with intracellular chaperones.

Disparate binding of chaperone proteins by HLA-A subtypes

We examined chaperone association with subtypes of HLA-A68 differing at positions 116 and/or 70, and analyzed the surface expression of each A68 subtype. Our findings with A68 indicate that certain subtypes have inefficient association with the assembly complex and correspondingly high surface expression, dependent on the character of position 116. Specifically, poor association of A68 subtypes with the transporter associated with antigen processing correlated with a comparatively high level of W6/32(+) forms at the cell surface. This observation suggests that intracellular retention is a dominant function of the assembly complex and that natural differences in assembly complex interaction may dictate the level of surface expression of MHC class I molecules. We also found that position 116 was crucial for HLA-A68 subtype association with the assembly complex. Our data contrast with results we obtained previously with HLA-B7 in that an aspartic acid at position 116 abrogated chaperone association for HLA-A68, whereas it increased association for HLA-B7. In total, HLA-A molecules exhibit natural allele-specific distinctions in chaperone association that correlate with differences in cell surface expression and with the identity of amino acid position 116.

Calreticulin binds to the alpha1 domain of MHC class I independently of tapasin

Prior to binding to antigenic peptide, the major histocompatibility complex (MHC) heavy chain associates with an assembly complex of proteins that includes calreticulin, tapasin, and the transporter associated with antigen processing (TAP). Our data show that calreticulin can bind weakly to Ld without tapasin's assistance, and that deglycosylation of the alpha1 domain results in a primary loss of binding to calreticulin rather than tapasin. We have also shown that high amounts of wild-type tapasin are still unable to associate with MHC class I in the absence of the MHC class I/calreticulin interaction, confirming the central role of calreticulin in the formation of the MHC class I assembly complex.

A region of tapasin that affects L(d) binding and assembly

Tapasin has been shown to stabilize TAP and to link TAP to the MHC class I H chain. Evidence also has been presented that tapasin influences the loading of peptides onto MHC class I. To explore the relationship between the ability of tapasin to bind to TAP and the MHC class I H chain and the ability of tapasin to facilitate class I assembly, we have created novel tapasin mutants and expressed them in 721.220-L(d) cells. One mutant has a deletion of nine amino acid residues (tapasin Delta334-342), and the other has amino acid substitutions at positions 334 and 335. In this report we describe the ability of these mutants to interact with L(d) and their effects on L(d) surface expression. We found that tapasin Delta334-342 was unable to bind to the L(d) H chain, and yet it facilitated L(d) assembly and expression. Tapasin Delta334-342 was able to bind and stabilize TAP, suggesting that TAP stabilization may be important to the assembly of L(d). Tapasin mutant H334F/H335Y, unlike tapasin Delta334-342, bound to L(d). Expression of tapasin H334F/H335Y in 721.220-L(d) reduced the proportion of cell surface open forms of L(d) and retarded the migration of L(d) from the endoplasmic reticulum. In total, our results indicate that the 334-342 region of tapasin influences L(d) assembly and transport.