An AGM Model for Changes in Complement during Pregnancy: Neutralization of Influenza Virus by Serum Is Diminished in Late Third Trimester

Abstract

Pregnant women in the third trimester are at increased risk of severe influenza disease relative to the general population, though mechanisms behind this are not completely understood. The immune response to influenza infection employs both complement (C′) and antibody (Ab). The relative contributions of these components to the anti-viral response are difficult to dissect because most humans have pre-existing influenza-specific Abs. We developed the African green monkey (AGM) as a tractable nonhuman primate model to study changes in systemic innate immunity to influenza during pregnancy. Because the AGMs were influenza-naïve, we were able to examine the role of C′ in influenza virus neutralization using serum from non-pregnant animals before and after influenza infection. We determined that serum from naïve AGMs neutralized influenza via C′, while post-infection neutralization did not require C′, suggesting an Ab-mediated mechanism. The latter mimicked neutralization using human serum. Further, we found that ex vivo neutralization of influenza with both naïve and influenza-immune AGM serum occurred by virus particle aggregation and lysis, with immune serum lysing virus at a much higher rate than naïve serum. We hypothesized that the anti-influenza C′ response would diminish late in AGM pregnancy, corresponding with the time when pregnant women suffer increased influenza severity. We found that influenza neutralization capacity is significantly diminished in serum collected late in the third trimester. Strikingly, we found that circulating levels of C3, C3a, and C4 are diminished late in gestation relative to nonpregnant animals, and while neutralization capacity and serum C3a return to normal shortly after parturition, C3 and C4 levels do not. This AGM model system will enable further studies of the role of physiologic and hormonal changes in downregulating C′-mediated anti-viral immunity during pregnancy, and it will permit the identification of therapeutic targets to improve outcomes of influenza virus infection in pregnant women.

The complement system and adverse pregnancy outcomes

“…This study was consistent with an earlier study by Richani demonstrating increased C3a and C5a during pregnancy (Richani et al, 2005). In contrast, in an African green monkey model of influenza during pregnancy, investigators found that C3, C3a and C4 were actually decreased late in gestation compared to non pregnant animals, correlating with decreased viral neutralization and influenza susceptibility late in pregnancy (Mayer and Parks, 2014). Clearly when considering complement and its activation in adverse pregnancy outcomes, a gestationally matched control group of pregnant women with no adverse outcomes must be used rather than simply using non-pregnant controls.…”

Influenza in High-Risk Hosts—Lessons Learned from Animal Models

“…Influenza infection in the pregnant ferret model does show reduced IgG-specific B-cell responses after experimental infection, although it remains to be determined if this occurs during natural infection in humans (Yoon et al 2018). Interestingly, the complement pathway components C3, C3a, and C4 show diminished antibody-dependent neutralization of influenza in pregnant African green monkeys (Mayer and Parks 2014).…”

Role of hormones in the pregnancy and sex‐specific outcomes to infections with respiratory viruses*

“…This is in part due to a lack of animal models that recapitulate the natural progression of human IAV disease, endocrinology, and pregnancy. Nonhuman primates are an excellent model of pregnancy 319 and viral respiratory infections, [319][320][321][322][323][324][325] but most have differences in the hormonal cascade associated with parturition (labor onset). A combination of in vitro and in vivo models is necessary to dissect the complex interactions between sex hormones, pregnancy, and the pathogenesis of viral respiratory infections.…”