Dendritic cells initiate immune control of epstein-barr virus transformation of B lymphocytes in vitro.

The initiation of cell-mediated immunity to Epstein-Barr virus (EBV) has been analyzed with cells from EBV-seronegative blood donors in culture. The addition of dendritic cells (DCs) is essential to prime naive T cells that recognize EBV-latent antigens in enzyme-linked immunospot assays for interferon gamma secretion and eradicate transformed B cells in regression assays. In contrast, DCs are not required to control the outgrowth of EBV-transformed B lymphocytes from seropositive donors. Enriched CD4+ and CD8+ T cells mediate regression of EBV-transformed cells in seronegative and seropositive donors, but the kinetics of T-dependent regression occurs with much greater speed with seropositives. EBV infection of DCs cannot be detected by reverse transcription-polymerase chain reaction with primers specific for mRNA for the EBNA1 U and K exons. Instead, DCs capture B cell debris and generate T cells specific for EBV latency antigens. We suggest that the cross-presentation of EBV-latent antigens from infected B cells by DCs is required for the initiation of EBV-specific immune control in vivo and that future EBV vaccine strategies should target viral antigens to DCs.

Dendritic cells expand Epstein Barr virus specific CD8+ T cell responses more efficiently than EBV transformed B cells.

Adoptive transfer of Epstein Barr virus (EBV) specific cytotoxic T lymphocytes (CTLs) has been successfully applied in the treatment of EBV associated post-transplant lymphoproliferative disease (PTLD). In most studies EBV transformed B cells (LCLs) have been used for the induction of EBV specific T cell lines. Application of this approach to other EBV associated tumors is difficult, because LCLs focus T cell expansion toward immunodominant EBV antigens that are not expressed in EBV associated Hodgkin's lymphoma and nasopharyngeal carcinoma. Therefore, we compared dendritic cells (DCs) with LCLs for CD8+ T cell stimulation against dominant and subdominant EBV antigens. DCs expanded tenfold more EBNA3A and LMP2 specific CD8+ T cells than LCL and also stimulated EBV specific CTL from PTLD patients. Both, DCs and LCLs stimulations led to the expansion of high affinity T cells, capable to target EBV transformed B cells. While LCLs and DCs expressed MHC class I and II products at similar levels, DCs showed a higher expression of costimulatory and adhesion molecules. This resulted in more efficient T cell conjugate formation with DCs than with LCLs. We propose the use of DCs for stimulation of EBV specific T cells in active or passive immunotherapy of EBV associated malignancies.

CD4+ T cell responses in the immune control against latent infection by Epstein-Barr virus.

The human gamma-herpesvirus Epstein-Barr virus establishes latent, life-long infection in more than 95% of the human adult population. Despite its growth transforming capacity, most carriers control EBV associated malignancies efficiently and remain free of EBV+ tumors. It is commonly accepted that lymphoblastoid cells, expressing all EBV latent antigens, are targeted by the immune system and cause tumors only in immune-suppressed individuals. However, immune control of EBV associated malignancies which express only three or one EBV latent antigen is less obvious. Recent studies have addressed the pattern of EBV latent infection in healthy EBV carriers and the identity of EBV derived target antigens for CD4+ T cells. The results suggest that immune surveillance also extends to tumors, which have down-regulated most EBV latent antigens and therefore escape EBV specific immune recognition at least in part. EBV specific immunity that targets these tumors in healthy EBV carriers seems to fail specifically during the development of Hodgkin's disease, nasopharyngeal carcinoma and Burkitt's lymphoma. These three EBV+ tumors appear to subdue EBV immunity against the remaining EBV latent antigens in different ways or profit from the effect of other pathogens on EBV specific immune responses, when they develop in otherwise immune competent individuals. While immune control and immune escape of these so-called spontaneously arising EBV associated malignancies is just beginning to be understood, immune control of persisting EBV infection can serve as a model for tumor immune surveillance in general.

Endogenous MHC class II processing of a viral nuclear antigen after autophagy.

CD4+ T cells classically recognize antigens that are endocytosed and processed in lysosomes for presentation on major histocompatibility complex (MHC) class II molecules. Here, endogenous Epstein-Barr virus nuclear antigen 1 (EBNA1) was found to gain access to this pathway by autophagy. On inhibition of lysosomal acidification, EBNA1, the dominant CD4+ T cell antigen of latent Epstein-Barr virus infection, slowly accumulated in cytosolic autophagosomes. In addition, inhibition of autophagy decreased recognition by EBNA1-specific CD4+ T cell clones. Thus, lysosomal processing after autophagy may contribute to MHC class II-restricted surveillance of long-lived endogenous antigens including nuclear proteins relevant to disease.

Epstein-Barr nuclear antigen 1-specific CD4(+) Th1 cells kill Burkitt's lymphoma cells.

The gamma-herpesvirus, EBV, is reliably found in a latent state in endemic Burkitt's lymphoma. A single EBV gene product, Epstein-Barr nuclear Ag 1 (EBNA1), is expressed at the protein level. Several mechanisms prevent immune recognition of these tumor cells, including a block in EBNA1 presentation to CD8(+) killer T cells. Therefore, no EBV-specific immune response has yet been found to target Burkitt's lymphoma. We now find that EBNA1-specific, Th1 CD4(+) cytotoxic T cells recognize Burkitt's lymphoma lines. CD4(+) T cell epitopes of EBNA1 are predominantly found in the C-terminal, episome-binding domain of EBNA1, and approximately 0.5% of peripheral blood CD4(+) T cells are specific for EBNA1. Therefore, adaptive immunity can be directed against Burkitt's lymphoma, and perhaps this role for CD4(+) Th1 cells extends to other tumors that escape MHC class I presentation.

Distinct roles of IL-12 and IL-15 in human natural killer cell activation by dendritic cells from secondary lymphoid organs.

Dendritic cells (DCs) are known to induce the growth and function of natural killer (NK) cells. Here, we address the capacity of DCs to interact with NK cells in human lymphoid organs and identify the role of specific DC-derived cytokines. We demonstrate that DCs colocalize with NK cells in the T cell areas of lymph nodes. In culture, DCs from either blood or spleen primarily stimulate the CD56(bright)CD16- NK cell subset, which is enriched in secondary lymphoid tissues. Blocking of IL-12 abolished DC-induced IFN-gamma secretion by NK cells, whereas membrane-bound IL-15 on DCs was essential for NK cell proliferation and survival. Maturation by CD40 ligation promoted the highest IL-15 surface presentation on DCs and led to the strongest NK cell proliferation induced by DCs. These results identify secondary lymphoid organs as a potential DC/NK cell interaction site and identify the distinct roles for DC-derived IL-12 and IL-15 in NK cell activation.