Chronic, active Epstein-Barr virus infection.

Chronic active Epstein-Barr virus (EBV) infection is an uncommon outcome of EBV infection and may present as a waxing and waning or fulminant syndrome. Unlike acute infectious mononucleosis, wherein EBV establishes lifelong infection and survives by maintaining a delicate balance with the host as a latent infection, in chronic active EBV infection the host-virus balance is disturbed. The mechanisms by which this balance becomes perturbed are likely to be heterogenous and may involve host immune factors, viral factors, or both. A number of subtle immunologic defects have been reported in patients with chronic active EBV infection. Enhanced expression of viral genes has also been noted in some cases. Treatment of chronic active EBV infection has proven difficult, but new modalities including etoposide-based regimens and adoptive transfer of EBV-specific cytotoxic T lymphocytes have shown promise.

Analysis of the mutant HLA-A*0201 heavy chain H74L: impaired TAP-dependent peptide loading.

A mutation of the HLA-A*0201 heavy chain at position 74 from histidine to leucine (H74L) resulted in a molecule with an interesting phenotype. H74L-expressing targets were recognized by peptide-specific HLA-A*0201-restricted cytotoxic T lymphocytes at lower peptide concentrations than wild type HLA-A*0201. H74L's improved ability to sensitize cells for tysis was due to its enhanced capability to bind exogenous peptide. Furthermore, this phenotype of improved exogenous binding and functional recognition was not peptide-specific. In contrast, the H74L molecule failed to present the HIV- HLA-A2-restricted pol peptide when expressed and processed endogenously. The inability to bind endogenous pol could be rescued by preceding the pol peptide with a signal sequence. The defect affecting endogenous presentation, therefore, appeared to be limited to the TAP-dependent pathway. Surprisingly, the H74L heavy chain was able to enter the defined MHC class I pathway and associate with beta2M, calreticulin, tapasin, and TAP. Despite the presence of the H74L heavy chain at the TAP complex, H74L was functionally inefficient at loading TAP-dependent peptides. H74L may help elucidate further steps in the process of loading TAP-dependent peptides into the class I cleft.

A point mutation in HLA-A*0201 results in failure to bind the TAP complex and to present virus-derived peptides to CTL.

Mutating the HLA-A*0201 heavy chain from threonine to lysine at position 134 (T134K) results in a molecule that presents exogenous peptide, but cannot present endogenously derived antigen. This is reflected in diminished cell surface expression and altered intracellular trafficking of T134K. The failure of T134K to present endogenous antigen can be overcome by using an ER targeting sequence, suggesting that the antigen presentation defect is restricted to TAP-dependent peptide loading. The ability of T134K to load peptide in a TAP-dependent manner is dramatically reduced compared with HLA-A*0201. By coimmunoprecipitation there is no detectable association of the T134K molecule with the TAP complex. Thus, T134K selectively affects TAP association and peptide loading, suggesting a requirement for the direct interaction of MHC class I heavy chain and the TAP complex for efficient presentation of endogenous antigen.

Genetic evidence for difference between intracellular and extracellular peptides in influenza A matrix peptide-specific CTL recognition.

During the course of extensive mutagenesis of HLA-A2.1, we examined influenza A matrix peptide (FMP)-specific CTL recognition of HMy2.C1R (C1R) cells expressing mutant HLA-A2.1 molecules, sensitized with synthetic peptide, FMP 58-66, (exogenous peptide), or infected with influenza A virus (endogenous peptide). Most mutants showed equivalent presentation of exogenous and endogenous peptides to FMP-specific CTL. However, five of the mutants differed in this property. Two of the five mutants, F9L and T134K, present exogenous peptide to FMP-specific CTL, but fail to present endogenous peptide to CTL. Western blot analysis using anti-matrix protein Ab indicates that the matrix protein is expressed in these mutants after infection with virus. Interestingly, transfection of these two mutants with a minigene encoding FMP 58-66 results in efficient lysis by FMP-specific CTL. Peptide-binding assays demonstrate that the two mutations dramatically decrease the binding of FMP. However, these mutants bind FMP as well as wild type in the presence of exogenously added human beta 2-m, suggesting that the lower affinity for beta 2-m leads to the inability to present endogenous peptide. The remaining three mutants, Y27N, Q32K, and S132C, fail to present exogenous peptide, but present endogenous peptide to FMP-specific CTL. Pulse-chase analyses followed by endoglycosidase-H treatment show that the rate of maturation and processing of the five mutant HLA-A2 molecules in C1R cells is identical to that of wild type. Overall, this study suggests that the assembly and subsequent recognition of endogenous peptide differs from that of exogenous peptide.