Sequence of Pathogenic Events in Cynomolgus Macaques Infected with Aerosolized Monkeypox Virus

Abstract

To evaluate new vaccines when human efficacy studies are not possible, the FDA's "Animal Rule" requires well-characterized models of infection. Thus, in the present study, the early pathogenic events of monkeypox infection in nonhuman primates, a surrogate for variola virus infection, were characterized. Cynomolgus macaques were exposed to aerosolized monkeypox virus (10 5 PFU). Clinical observations, viral loads, immune responses, and pathological changes were examined on days 2, 4, 6, 8, 10, and 12 postchallenge. Viral DNA (vDNA) was detected in the lungs on day 2 postchallenge, and viral antigen was detected, by immunostaining, in the epithelium of bronchi, bronchioles, and alveolar walls. Lesions comprised rare foci of dysplastic and sloughed cells in respiratory bronchioles. By day 4, vDNA was detected in the throat, tonsil, and spleen, and monkeypox antigen was detected in the lung, hilar and submandibular lymph nodes, spleen, and colon. Lung lesions comprised focal epithelial necrosis and inflammation. Body temperature peaked on day 6, pox lesions appeared on the skin, and lesions, with positive immunostaining, were present in the lung, tonsil, spleen, lymph nodes, and colon. By day 8, vDNA was present in 9/13 tissues. Blood concentrations of interleukin 1ra (IL-1ra), IL-6, and gamma interferon (IFN-␥) increased markedly. By day 10, circulating IgG antibody concentrations increased, and on day 12, animals showed early signs of recovery. These results define early events occurring in an inhalational macaque monkeypox infection model, supporting its use as a surrogate model for human smallpox. IMPORTANCEBioterrorism poses a major threat to public health, as the deliberate release of infectious agents, such smallpox or a related virus, monkeypox, would have catastrophic consequences. The development and testing of new medical countermeasures, e.g., vaccines, are thus priorities; however, tests for efficacy in humans cannot be performed because it would be unethical and field trials are not feasible. To overcome this, the FDA may grant marketing approval of a new product based upon the "Animal Rule," in which interventions are tested for efficacy in well-characterized animal models. Monkeypox virus infection of nonhuman primates (NHPs) presents a potential surrogate disease model for smallpox. Previously, the later stages of monkeypox infection were defined, but the early course of infection remains unstudied. Here, the early pathogenic events of inhalational monkeypox infection in NHPs were characterized, and the results support the use of this surrogate model for testing human smallpox interventions.

Improving the Care and Treatment of Monkeypox Patients in Low-Resource Settings: Applying Evidence from Contemporary Biomedical and Smallpox Biodefense Research

“…Because the intent is to replicate severe disease, MPXV infection studies using NHPs are rarely performed with low-dose virus challenge, and the dose required for successful infection can vary between different species of host. Both observational and experimental studies suggest that dose influences not only illness severity, but also the length of incubation period and the manifestations of illness in NHPs and in humans [ 42 , 55 , 56 ]. Respiratory challenge at high doses generally results in compressed incubation periods and respiratory syndromes and as such may have more bearing on the clinical picture of persons affected during MPX outbreaks during periods of significant inter-human transmission (i.e., transmission through respiratory routes).…”

Challenges and Achievements in Prevention and Treatment of Smallpox

“…Various models have been established throughout the years in various hosts. Non-human primates develop severe disease with disseminated lesions and death, following respiratory infections, yet a rather high dose of MPXV (doses of >10 6 pfu) is usually needed to produce acute severe systemic disease [ 96 , 97 , 98 ]. This potential disadvantage of the non-human primate model and the inherent complexities of working with NHPs led to the development of MPXV models in small animal species, including prairie dogs, squirrels, African dormice, Gambian rats, and selected mouse strains with attenuated immune response [ 17 , 99 , 100 , 101 , 102 , 103 , 104 ].…”

Animal Models of Human Viral Diseases

“…A recent study also evaluated the cytokine levels in aerosol challenged animals. (Tree et al, 2015). Tree et al (2015) showed that IFNγ, Il-1rα, and Il-6 increased dramatically on day 8 postexposure the day that death was most likely to occur, and viral DNA was detected in most of the tissues.…”

“…(Tree et al, 2015). Tree et al (2015) showed that IFNγ, Il-1rα, and Il-6 increased dramatically on day 8 postexposure the day that death was most likely to occur, and viral DNA was detected in most of the tissues. These results support the idea of a cytokine storm causing mortality in monkeypox disease.…”