Despite antiretroviral therapy (ART), HIV-1 persists in a stable latent reservoir1, 2, primarily in resting memory CD4+ T cells3, 4. This reservoir presents a major barrier to the cure of HIV-1 infection. To purge the reservoir, pharmacological reactivation of latent HIV-1 has been proposed5 and tested both in vitro and in vivo6–8. A key remaining question is whether virus-specific immune mechanisms including cytolytic T lymphocytes (CTL) can clear infected cells in ART-treated patients after latency is reversed. Here we show that there is a striking all or none pattern for CTL escape mutations in HIV-1 Gag epitopes. Unless ART is started early, the vast majority (>98%) of latent viruses carry CTL escape mutations that render infected cells insensitive to CTLs directed at common epitopes. To solve this problem, we identified CTLs that could recognize epitopes from latent HIV-1 that were unmutated in every chronically infected patient tested. Upon stimulation, these CTLs eliminated target cells infected with autologous virus derived from the latent reservoir, both in vitro and in patient-derived humanized mice. The predominance of CTL-resistant viruses in the latent reservoir poses a major challenge to viral eradication. Our results demonstrate that chronically infected patients retain a broad spectrum viral-specific CTL response and that appropriate boosting of this response may be required for the elimination of the latent reservoir.

Many pathogens that cause human disease infect only humans. To identify the mechanisms of immune protection against these pathogens and also to evaluate promising vaccine candidates, a small animal model would be desirable. We demonstrate that primary T cell responses in mice with reconstituted human immune system components control infection with the oncogenic and persistent Epstein-Barr virus (EBV). These cytotoxic and interferon-γ–producing T cell responses were human leukocyte antigen (HLA) restricted and specific for EBV-derived peptides. In HLA-A2 transgenic animals and similar to human EBV carriers, T cell responses against lytic EBV antigens dominated over recognition of latent EBV antigens. T cell depletion resulted in elevated viral loads and emergence of EBV-associated lymphoproliferative disease. Both loss of CD4+ and CD8+ T cells abolished immune control. Therefore, this mouse model recapitulates features of symptomatic primary EBV infection and generates T cell–mediated immune control that resists oncogenic transformation.

Immunodeficient mice reconstituted with a human immune system represent a promising tool for translational research as they may allow modeling and therapy of human diseases in vivo. However, insufficient development and function of human natural killer (NK) cells and T cell subsets limit the applicability of humanized mice for studying cancer biology and therapy. Here, we describe a human interleukin 15 () and human signal regulatory protein alpha () knock-in mouse on a background (SRG-15). Transplantation of human hematopoietic stem and progenitor cells into SRG-15 mice dramatically improved the development and functional maturation of circulating and tissue-resident human NK and CD8 T cells and promoted the development of tissue-resident innate lymphoid cell (ILC) subsets. Profiling of human NK cell subsets by mass cytometry revealed a highly similar expression pattern of killer inhibitory receptors and other candidate molecules in NK cell subpopulations between SRG-15 mice and humans. In contrast to nonobese diabetic severe combined immunodeficient (NSG) mice, human NK cells in SRG-15 mice did not require preactivation but infiltrated a Burkitt's lymphoma xenograft and efficiently inhibited tumor growth following treatment with the therapeutic antibody rituximab. Our humanized mouse model may thus be useful for preclinical testing of novel human NK cell-targeted and combinatory cancer immunotherapies and for studying how they elicit human antitumor immune responses in vivo.

IntroductionEpstein-Barr virus (EBV) is a ubiquitous human ␥-herpesvirus that latently infects B cells and establishes chronic infection in more than 90% of the adult population. Although infection with EBV during adolescence can lead to infectious mononucleosis (IM), the vast majority of infected people acquires and harbors EBV as a benign lifelong infection, which is controlled by strong T-cell immunity. However, in a small subset of infected individuals, EBV latency programs with different viral antigen expression patterns are associated with malignancies such as Hodgkin and Burkitt lymphomas as well as nasopharyngeal carcinoma (NPC). 1 The nuclear antigen 1 (EBNA1) is the one EBV antigen that is expressed in all of these EBV-associated tumors as well as in EBV-positive proliferating cells in healthy carriers. 2 EBNA1 is crucial for viral persistence, because it initiates viral DNA replication and anchors the circular viral episome to the mitotic chromosomes during cell division, thereby ensuring the survival of the viral genome in proliferating cells. Thus, even in the absence of all other EBV proteins, such as in Burkitt lymphoma, EBNA1 expression must be maintained, and from an immune surveillance point of view, EBNA1 should be a critical target of protective immunity. Indeed, EBNA1 is consistently recognized by Th1-type CD4 ϩ T cells, [3][4][5][6] and can elicit CD8 ϩ T-cell responses 7-9 in healthy EBV carriers. These T cells that recognize mostly epitopes in the C-terminal domain of EBNA1 can target EBV-transformed B cells and prevent their outgrowth in vitro. 10 While EBNA1-specific T-cell responses can also be detected in peripheral blood of NPC patients, 11 they are greatly diminished in patients with EBVassociated non-Hodgkin lymphoma in the context of HIV infection, 12 EBV-associated Hodgkin disease (K. N. Heller, F.A., P. Steinherz, C. Postlook, A. Chadburn, K. Kelly, C.M., manuscript submitted) and endemic Burkitt lymphomas (Ann Moormann, Case Western Reserve University, Cleveland, OH, personal communication April 2008), thus making EBNA1, and specifically its C-terminal domain, a logical target for vaccine development against all EBV-associated malignancies.A promising cell type, to which EBNA1 could be targeted for vaccine design, is dendritic cells (DCs). These sentinels of the immune system have an exceptional T-cell stimulatory capacity, which includes their ability to efficiently process antigens, and present them on both major histocompatibility class (MHC) I and class II molecules in combination with T-cell costimulatory molecules. 13 DCs are also crucial for initiating protective innate and adaptive immune responses against bacterial and viral pathogens in vivo, 14,15 which further supports targeting DCs for therapeutic vaccination. However, many current DC-targeted immunization approaches use individualized culture, antigen loading, and activation of DCs in vitro for adoptive transfer. 16 A more recent strategy that circumvents the analysis of ex vivo DCs is to target antigens to DCs in...