Baboon bone-marrow xenotransplant in a patient with advanced HIV disease: case report and 8-year follow-up

Stem cell transplantation in strategies for curing HIV/AIDS

HIV-1 can persist in a latent form in resting memory CD4+ cells and macrophages carrying an integrated copy of the HIV genome. Because of the presence of these stable reservoir cells, eradication by antiretroviral therapy is unlikely and in order to achieve eradication, alternative treatment options are required. Stem cell transplantation has been considered previously to effect the clinical course of HIV-infection but in practice eradication or virus control was not achievable. However, modifications of stem cell transplantation using natural or artificial resistant cell sources, combination with new techniques of gene editing or generating cytotoxic anti HIV effector cells have stimulated this field of HIV cell therapy substantially. Here, we look back on 30 years of stem cell therapy in HIV patients and discuss most recent developments in this direction.

Xenozoonoses

Immunological and technical advances have led to tremendous increases in the number of people potentially able to benefit from allotransplantation. Ironically, it is the success of the field that has led to a renewed interest in xenotransplantation during the past several decades. To a large part, this has occurred because of the great scarcity of human organ and tissue donors. However, it has expanded to include the use of cells from animals into humans such as porcine islet cells for diabetes or extracorporeal perfusion of human blood through animal organs or cells. Similar to allotransplantation, issues regarding transmission of infections from the graft to the human recipient were brought up for consideration with these procedures in the 1990s (Michaels and Simmons, 1994; Chapman et al., 1995; Hammel et al., 1998; Fishman et al., 1998). A risk for infection exists with the use of any biologic agent regardless of whether it is from a human or an animal source. Accordingly, transmission of infections from human organs, tissues, or cells is a well-recognized cause of disease after allotransplantation (Ison and Grossi, 2013; Green and Michaels, 2012). As the human graft shortage continues, newer cellular therapies are explored. Thus, attention continues to be given to the potential use of xenogeneic organs, tissues, or cells for human maladies through xenotransplantation. The potential for novel zoonotic infections to emerge because of xenotransplantation (xenozoonoses or xenosis) led to a debate on whether the field should be permitted to progress. This chapter reviews the issues of xenotransplantation related to infections from animals to humans. Lessons learned from infections with prior nonhuman primate xenotransplantation and human allotransplantation are used to help inform about risks with newer xenogeneic procedures. In addition, information on known zoonoses is reviewed to better develop constructs to decrease the hazard of infection with these novel procedures.

Potential deleterious role of anti-Neu5Gc antibodies in xenotransplantation

Human beings do not synthesize the glycolyl form of the sialic acid (Neu5Gc) and only express the acetylated form of the sugar, whereas a diet-based intake of Neu5Gc provokes a natural immunization and production of anti-Neu5Gc antibodies in human serum. However, Neu5Gc is expressed on mammal glycoproteins and glycolipids in most organs and cells. We review here the relevance of Neu5Gc and anti-Neu5Gc antibodies in the context of xenotransplantation and the use of animal-derived molecules and products, as well as the possible consequences of a long-term exposure to anti-Neu5Gc antibodies in recipients of xenografts. In addition, the importance of an accurate estimation of the anti-Neu5Gc response following xenotransplantation and the future contribution of knockout animals mimicking the human situation are also assessed.

Novel cell and gene therapies for HIV

Highly active antiretroviral therapy dramatically improves survival in HIV-infected patients. However, persistence of HIV in reservoirs has necessitated lifelong treatment that can be complicated by cumulative toxicities, incomplete immune restoration, and the emergence of drug-resistant escape mutants. Cell and gene therapies offer the promise of preventing progressive HIV infection by interfering with HIV replication in the absence of chronic antiviral therapy. Individuals homozygous for a deletion in the CCR5 gene (CCR5Δ32) are largely resistant to infection from R5-topic HIV-1 strains, which are most commonly transmitted. A recent report that an HIV-infected patient with relapsed acute myelogenous leukemia was effectively cured from HIV infection after transplantation of hematopoietic stem/progenitor cells (HSC) from a CCR5Δ32 homozygous donor has generated renewed interest in developing treatment strategies that target viral reservoirs and generate HIV resistance in a patient's own cells. Although the development of cell-based and gene transfer therapies has been slow, progress in a number of areas is evident. Advances in the fields of gene-targeting strategies, T-cell-based approaches, and HSCs have been encouraging, and a series of ongoing and planned trials to establish proof of concept for strategies that could lead to successful cell and gene therapies for HIV are under way. The eventual goal of these studies is to eliminate latent viral reservoirs and the need for lifelong antiretroviral therapy.

Baboon model for West Nile virus infection and vaccine evaluation

Animal models that closely mimic the human condition are of paramount significance to study pathogenic mechanisms, vaccine and therapy scenarios. This is particularly true for investigations that involve emerging infectious diseases. Nonhuman primate species represent an alternative to the more intensively investigated rodent animal models and in a number of instances have been shown to represent a more reliable predictor of the human response to infection. West Nile virus (WNV) has emerged as a new pathogen in the Americas. It has a 5% fatality rate, predominantly in the elderly and immune compromised. Typically, infections are cleared by neutralizing antibodies, which suggests that a vaccine would be efficacious. Previously, only macaques had been evaluated as a primate model for WNV vaccine design. The macaques did not develop WNV disease nor express the full complement of IgG subclasses that is found in humans. We therefore explored baboons, which exhibit the similar four IgG subclasses observed in humans as a new model for WNV infection and vaccine evaluation. In this present report, we describe the experimental infection of baboons with WNV and test the efficacy of an inactivated WNV vaccination strategy. All experimentally infected animals developed transient viremia and subsequent neutralizing antibodies. Anti-WNV IgM antibodies peaked at 20 days post-infection. Anti-WNV IgG antibodies appeared later and persisted past 60 days. Prior vaccination with chemically inactivated virus induced neutralizing titers and a fast, high titer IgG recall response, which resulted in lower viremia upon challenge. This report is the first to describe the development of the baboon model for WNV experimental infection and the utility of this model to characterize the immunologic response against WNV and a candidate WNV vaccine.

Clinical experience with therapeutic AIDS vaccines

Recently, there has been a renewed interest in therapeutic vaccination as an adjunct or alternative to current treatment options for HIV. The first immunotherapeutic trial relevant to this topic was published in 1983. Since then, several dozen therapeutic vaccine trials have been carried out. The results have consistently shown that although in vitro-measured HIV-specific immune responses were evident as a result of vaccination, clinical improvement has been seldom observed. The instances of apparent clinical benefit however, were invariably associated with the usage of vaccines that acted in accord with the principles of allo- or autoimmunization. The majority of these vaccines were derived from the blood of HIV carriers or a cell culture and therefore inherently contained host-cell antigens unrelated to HIV. These observations raise the issue of whether this clinically successful approach has been unduly neglected. Most commercial vaccines on the market today are made the old-fashioned way, but very little support or attention has been given to the development of such vaccines for AIDS therapy. The current strategy, biased toward vaccines which have shown little evidence of clinical efficacy, is shortsighted and needs to be revised.