Simian retroviruses: Infection and disease—implications for immunotoxicology research in primates

Non-human primates have assumed an important role in preclinical safety assessment studies, particularly in the evaluation of biopharmaceutical and immunomodulatory therapies. Naturally occurring simian retrovirus infections may adversely affect the suitability of primates for use in such studies. Various species of non-human primates are the natural hosts for six exogenous retroviruses, representing five genera within the family Retroviridae. Retroviruses establish persistent infections with a broad spectrum of pathogenic potential, ranging from nonpathogenic to highly pathogenic, depending on the variety of the host, virus, and environmental factors. In the context of immunotoxicology, in which the research objective is to specifically evaluate the effect of drugs or biologics on the immune system, the immune modulatory effects of simian retroviruses, which may be subtle or profound, may introduce significant confounding into the studies of immunotoxic effects utilizing non-human primates. Latent or subclinical retrovirus infections are common and research-related procedures may lead to virus reactivation or overt disease. Adverse effects of undetected retrovirus infections on preclinical research include the loss of experimental subjects (and potentially of statistical power) due to increased morbidity and mortality, virus-induced clinical abnormalities, histologic lesions, alteration of physiologic parameters and biologic responses, and interference with in vitro assays and/or cytolytic destruction of primary cell cultures. The aim of this review is to provide an overview of the key biological, clinical, and pathological features of several important simian retroviruses, with emphasis on viruses infecting macaques and other primate species commonly used in preclinical research, and a discussion of the implications of these infections for immunotoxicology and other preclinical research in primates. Adequate pre-study retrovirus screening is essential to exclude retrovirus-infected primates from research protocols.