Long-Term Protection of Rhesus Macaques from Zika Virus Reinfection.

By the end of the 2016 Zika virus (ZIKV) outbreak, it is estimated that there were up to 100 million infections in the Americas. In approximately one in seven infants born to mothers infected during pregnancy, ZIKV has been linked to microcephaly, developmental delays, or other congenital disorders collectively known as congenital Zika syndrome, as well as Guillain-Barré syndrome, in ZIKV-infected adults. It is a global health priority to develop a vaccine against ZIKV that elicits long-lasting immunity; however, the durability of immunity to ZIKV is unknown. Previous studies in mice and nonhuman primates have been crucial in vaccine development but have not defined the duration of immunity generated by ZIKV infection. In this study, we rechallenged five rhesus macaques with ZIKV 22 to 28 months after a primary ZIKV infection. We show that primary ZIKV infection generates high titers of neutralizing antibodies that protect from detectable plasma viremia following rechallenge and persist for at least 22 to 28 months. While additional longitudinal studies are necessary with longer time frames, this study establishes a new experimentally defined minimal length of protective ZIKV immunity.IMPORTANCE ZIKV emerged as a vector-borne pathogen capable of causing illness in infected adults and congenital birth defects in infants born to mothers infected during pregnancy. Despite the decrease in ZIKV cases since the 2015-2016 epidemic, questions concerning the prevalence and longevity of protective immunity have left vulnerable communities fearful that they may become the center of next ZIKV outbreak. Although preexisting herd immunity in regions of past outbreaks may dampen the potential for future outbreaks to occur, we currently do not know the longevity of protective immunity to ZIKV after a person becomes infected. Here, we establish a new experimentally defined minimal length of protective ZIKV immunity. We show that five rhesus macaques initially infected with ZIKV 22 to 28 months prior to rechallenge elicit a durable immune response that protected from detectable plasma viremia. This study establishes a new minimal length of protective immunity.

Highly efficient maternal-fetal Zika virus transmission in pregnant rhesus macaques.

Infection with Zika virus (ZIKV) is associated with human congenital fetal anomalies. To model fetal outcomes in nonhuman primates, we administered Asian-lineage ZIKV subcutaneously to four pregnant rhesus macaques. While non-pregnant animals in a previous study contemporary with the current report clear viremia within 10-12 days, maternal viremia was prolonged in 3 of 4 pregnancies. Fetal head growth velocity in the last month of gestation determined by ultrasound assessment of head circumference was decreased in comparison with biparietal diameter and femur length within each fetus, both within normal range. ZIKV RNA was detected in tissues from all four fetuses at term cesarean section. In all pregnancies, neutrophilic infiltration was present at the maternal-fetal interface (decidua, placenta, fetal membranes), in various fetal tissues, and in fetal retina, choroid, and optic nerve (first trimester infection only). Consistent vertical transmission in this primate model may provide a platform to assess risk factors and test therapeutic interventions for interruption of fetal infection. The results may also suggest that maternal-fetal ZIKV transmission in human pregnancy may be more frequent than currently appreciated.

Neutrophil progenitor populations of rhesus macaques.

Captive-bred rhesus macaques of Indian origin represent one of the most important large animal models for infectious disease, solid organ transplantation, and stem cell research. There is a dearth of information defining hematopoietic development, including neutrophil leukocyte differentiation in this species using multicolor flow cytometry. In the current study, we sought to identify cell surface markers that delineate neutrophil progenitor populations with characteristic immunophenotypes. We defined four different postmitotic populations based on their CD11b and CD87 expression pattern, and further refined their immunophenotypes using CD32, CD64, lactoferrin, and myeloperoxidase as antigenic markers. The four subsets contained myelocyte, metamyelocyte, band, and segmented neutrophil populations. We compared our flow cytometry-based classification with the classical nuclear morphology-based classification. We found overlap of immunological phenotype between populations of different nuclear morphology and identified phenotypically different subsets within populations of similar nuclear morphology. We assessed the responsiveness of these populations to stimulatory signals, such as LPS, fMLP, or PMA, and demonstrated significant differences between human and rhesus macaque neutrophil progenitors. In this study, we provided evidence for species-specific features of granulopoiesis that ultimately manifested in the divergent immunophenotypes of the fully differentiated segmented neutrophils of humans and rhesus macaques. Additionally, we found functional markers that can be used to accurately quantify neutrophil progenitors by flow cytometry. Although these markers do not coincide with the classical nuclear-morphology-based grading, they enable us to perform functional studies monitoring immunophenotypic markers.

Heterologous Protection against Asian Zika Virus Challenge in Rhesus Macaques.

BACKGROUND: Zika virus (ZIKV; Flaviviridae, Flavivirus) was declared a public health emergency of international concern by the World Health Organization (WHO) in February 2016, because of the evidence linking infection with ZIKV to neurological complications, such as Guillain-Barre Syndrome in adults and congenital birth defects including microcephaly in the developing fetus. Because development of a ZIKV vaccine is a top research priority and because the genetic and antigenic variability of many RNA viruses limits the effectiveness of vaccines, assessing whether immunity elicited against one ZIKV strain is sufficient to confer broad protection against all ZIKV strains is critical. Recently, in vitro studies demonstrated that ZIKV likely circulates as a single serotype. Here, we demonstrate that immunity elicited by African lineage ZIKV protects rhesus macaques against subsequent infection with Asian lineage ZIKV. METHODOLOGY/PRINCIPAL FINDINGS: Using our recently developed rhesus macaque model of ZIKV infection, we report that the prototypical ZIKV strain MR766 productively infects macaques, and that immunity elicited by MR766 protects macaques against heterologous Asian ZIKV. Furthermore, using next generation deep sequencing, we found in vivo restoration of a putative N-linked glycosylation site upon replication in macaques that is absent in numerous MR766 strains that are widely being used by the research community. This reversion highlights the importance of carefully examining the sequence composition of all viral stocks as well as understanding how passage history may alter a virus from its original form. CONCLUSIONS/SIGNIFICANCE: An effective ZIKV vaccine is needed to prevent infection-associated fetal abnormalities. Macaques whose immune responses were primed by infection with East African ZIKV were completely protected from detectable viremia when subsequently rechallenged with heterologous Asian ZIKV. Therefore, these data suggest that immunogen selection is unlikely to adversely affect the breadth of vaccine protection, i.e., any Asian ZIKV immunogen that protects against homologous challenge will likely confer protection against all other Asian ZIKV strains.