Type 2 cytokines predominate in the human CD4+ T-lymphocyte response to Epstein-Barr virus nuclear antigen 1.

Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus that infects more than 85% of the adult population worldwide and establishes a persistent infection for the lifetime of the host. The EBV carrier state (latency) is characterized by expression of a limited set of EBV genes and by sporadic reactivation of the lytic cycle in latently infected cells, leading to viral replication. Several lines of evidence implicate latently infected B lymphocytes as the major EBV reservoir. Upon reactivation in tonsillar B lymphocytes, EBV can productively infect oropharyngeal epithelium, resulting in infectious virus production and transmission (29, 49). The importance of EBV as a human pathogen is evinced by its etiologic role in the infectious mononucleosis syndrome. The potential significance of EBV in malignancy is suggested by its B-cell-transforming ability in vitro and by its strong association with a number of human malignancies, including Burkitt's lymphoma, nasopharyngeal carcinoma, Hodgkin's disease, and immunoblastic lymphomas in immunocompromised patients. Despite the oncogenic potential of the virus, the vast majority of EBV-infected individuals remain asymptomatic. A considerable amount of evidence suggests that cell-mediated immune response, particularly CD8+ cytotoxic T-lymphocyte (CTL) recognition of the EBV latent proteins in persistently infected cells, is critical in suppressing EBV replication in latently infected individuals (reviewed in references 49 and 50). Studies of EBV latent gene expression in persistently infected cells and EBV-positive tumors including both spontaneous and induced EBV-transformed B-lymphoblastoid cell lines (LCL) suggest that up to eight EBV proteins including six nuclear antigens (i.e., EBV nuclear antigen 1 [EBNA1], -2, -3A, -3B, -3C, and -LP) and two membrane proteins (latent membrane protein 1 [LMP1] and 2 [LMP2]) may be differentially expressed in cells exhibiting various forms of EBV latency. Significantly, EBNA1 is the only EBV latent antigen consistently expressed in all patterns of EBV latent gene expression, including EBV-positive malignancies; this reflects its essential function in maintaining the viral genome on latently infected cells. However, the dominant major histocompatibility complex (MHC) class I-restricted CTL responses identified in latently infected EBV carriers are directed to the EBNA3 family of latent gene products. Subdominant reactivities to LMP1 and LMP2, as well as to EBNA2 and EBNALP, have also been found in several individuals (50). By contrast, with rare exceptions (9), MHC class I-restricted CTL responses have not been detected against EBNA1 in EBV-seropositive carriers in spite of the requirement for EBNA1 expression during latent infection. The likely mechanism responsible for this absence of MHC class I-restricted CTL responses against EBNA1 in latently infected cells is cis-acting interference with proteosomal processing of EBNA1 mediated by the internal glycine-alanine repeat domain of EBNA1 (32, 33, 55). This finding led to speculation that inhibition by EBNA1 of its presentation to CD8+ MHC class I-restricted T lymphocytes may account for the lack of elimination of latently infected B lymphocytes in healthy carriers, and hence for EBV persistence and latent infection in healthy carriers (26, 30, 35, 42). In contrast to the large body of data on the response of MHC class I-restricted CD8+ T lymphocytes to EBV latent proteins, minimal data are available on the response of MHC class II-restricted CD4+ T lymphocytes to these EBV gene products. In view of the importance of EBNA1 in EBV latency and persistence and the lack of a significant MHC class I-restricted CD8+ T-cell response to this latent antigen, we examined the response of CD4+ T lymphocytes to this essential EBV antigen to establish the contribution of EBNA1-specific MHC class II-restricted T lymphocytes to the control of EBV latency. In parallel with the analysis of EBNA1-specific T lymphocytes, we simultaneously examined the response of peripheral blood mononuclear cells (PBMC) from EBV-positive donors to another EBV latent antigen, EBNA3C. In contrast to EBNA1, the EBNA3C antigen represents a dominant target of the vigorous EBV-specific MHC class I-restricted cytolytic CD8+ T-cell response detected in healthy viral carriers (10, 26, 42, 50, 51). In this report, we demonstrate that EBNA1-specific CD4+ T lymphocytes from the peripheral blood of a panel of unrelated healthy donors mount a vigorous proliferative response and preferentially secrete type 2 cytokines in response to the EBNA1 antigen. By contrast, EBNA3C-specific CD4+ T lymphocytes from these donors exhibit a characteristic type 1 cytokine response to this antigen. The significance of this unique cytokine response to EBNA1 for the maintenance of EBV latency and the development of EBV-related malignancies is discussed.