Cytomegalovirus (CMV) & Maternal Immunity

Cytomegalovirus (CMV)

• Cytomegalovirus (CMV) leads to congenital infection and birth defects
• Primary CMV infection during pregnancy can result in congenital CMV (cCMV) more frequently
• Maternal immunity may provide partial protection
• Limited knowledge of protective immune responses impacts vaccine development

Study Overview

• Explored maternal plasma rhesus CMV (RhCMV) viral load (VL)
• Explored RhCMV-specific antibody binding and functional responses
• Subjects were 12 immunocompetent dams with primary RhCMV infection
• CMV transmission was defined by RhCMV detection in amniotic fluid (AF) using qPCR
• Using combined data from past and current primary RhCMV infection, studies compared RhCMV AF-positive and negative dams
• Study included immunocompetent dams, CD4+ T cell-depleted dams with and without RhCMV-specific polyclonal IgG infusion

Key Findings

• Maternal plasma RhCMV viral load was higher in AF-positive dams during the initial 3 weeks post-infection
• RhCMV glycoprotein B (gB)- and pentamer-specific IgG responses were lower compared to AF-negative dams
• The CD4+ T cell depletion drove observed disparities

Implications

• Maternal viremia and humoral responses are not associated with cCMV in healthy individuals
• Innate immunity may play an important role
• Antibody responses during acute infection might develop too late to affect vertical transmission
• Pre-existing CMV glycoprotein-specific and neutralizing IgG could protect against cCMV in high-risk, immunocompromised cases

Author Conclusions

• Maternal plasma virus levels do not predict AF transmission in immunocompetent dams
• CD4+ T cell depleted macaques with virus in AF: higher plasma viral loads than dams without placental transmission
• Virus-specific antibody responses are too slow to prevent congenital transmission after maternal infection
• Vaccines must confer protective immunity levels to CMV-naïve mothers before pregnancy

Introduction

• Congenital CMV is the most common infectious cause of hearing loss in children
• Placental transmission is more common after primary human CMV (HCMV) infection
• Biological factors that determine why some CMV negative women contract primary infection during pregnancy which results in congenital infection are ill-defined
• The study looks at maternal humoral immune responses in vertical CMV transmission to guide vaccine development

Methods for Overcoming Logistical Hurdles

• In humans, CMV infection is hard to detect due to being asymptomatic
• HCMV cannot infect animal models
• Non-human primates (NHPs) closely resemble humans in terms of anatomy and physiology of pregnancy
• Continuous immunological arms race between CMV and host species resulted in co-adaptation, evidenced by phylogenetic trees and strict species specificity
• RhCMV share the most recent common ancestor and conservation of coding ORFs and gene families, viral immune evasion strategies, and viral pathogenesis with HCMV.
• A rhesus macaque model to study maternal CMV infection in early pregnancy that is similar to the first and second trimesters of gestation, where congenital transmission occurs in subsets of animals was developed
• High maternal viremia is associated with vertical RhCMV transmission, so increased virus load at interface increases placental infection and congenital transmission
• Maternal virus-specific functional antibody responses prevent vertical transmission

Study Design and Antibody Response Characterization

• Pregnant NHP groups challenged with RhCMV at the end of the first/early second trimester
• Model vertical CMV transmission: immunocompetent dams, CD4 T cell-depleted
• CD4+ T cell-depleted dams also received polyclonal RhCMV-specific IgG infusion
• The antibody responses in acutely RhCMV-infected dams, like ADCP and ADCC functions, were measured
• Responses and maternal VL were analyzed, with no significant differences in immunocompetent dams
• RhCMV glycoprotein-specific binding responses were higher in CD4-depleted animals with passive IgG infusion

Key Results on RhCMV Vertical Transmission

• RhCMV vertical transmission was confirmed in some immunocompetent dams post infection
• To prove this 12 RhCMV-seronegative dams cells were injected with epithelial-tropic, low passage isolate UCD52 RhCMV strain and bacterial artificial chromosome (BAC)-derived, clonal full-length (FL) RhCMV
• Then the viral load of maternal plasma and amniotic fluid (AF) were Monitored
• Plasma VL peaked 1-2 weeks post infection in all the cells
• CMV DNA was detectable by qPCR in AF among cells
• AF virus was detected within 4 weeks post infection in three dams
• Kinetics of the plasma VL was consistent in AF-positive versus AF-negative dams
• Plasma RhCMV-specific IgM responses were detectable at low levels
• IgG binding shared kinetics, with responses by gB and the pentameric complex (PC)

Effector Function Kinetics

• RhCMV-neutralizing responses in fibroblasts and epithelial cells developed weeks post infection
• Non-neutralizing Fc-mediated antibody effector responses, ADCP and ADCC, developed gradually over infection
• Antibody responses were similar in AF-positive and negative dams

Impact of Including Historical Groups

• Maternal infection, administration timing, total inoculum dose are similar among groups, but immunologic manipulations differ

Viral Load Analysis

• The maternal VL was not significantly different between AF-positive vs AF-negative dams in the immunocompetent group.
• In contrast, plasma VL was higher among AF-positive dams
• Differences in plasma VL by CD4-depleted group are confounded by the treatment group

Antibody Responses

• The measured RhCMV-specific binding or functional antibody responses were not statistically different in AF-positive versus AF-negative immunocompetent dams
• Plasma IgG binding to cell-associated gB and soluble PC were statistically associated with transmission status
• RhCMV glycoprotein- binding IgG responses was higher in dams by unadjusted Wilcoxon Rank Sum tests

Relationships Between Antibodies and Viral Load

• RhCMV-specific antibody responses show negative associations with maternal plasma VL
• Strong positive associations between IgG and IgM binding to whole UCD52 virions and positive associations between glycoprotein IgG binding and neutralization responses
• Principal component analysis (PCA) showed separation with maternal plasma VL. Clustering of CD4-depleted dams was observed
• High maternal anti-CMV ADCP responses might be associated with the protection from human vertical HCMV transmission

Study Conclusion

• Maternal plasma VL is not associated with transmitting dams
• VLs tended to have the animals fall into the middle of the distribution

Study Caveats

• Limited group sizes is A key caveat
• Including dams from prior studies introduces perturbations in humoral responses
• Non-human primates more closely resemble humans
• Much cross-reactivity between human and rhesus antibodies and Fc receptors
• Immune differences in the IgG subclasses that mediate effector functions impacts the animal model
• RhCMV contains related gene families to HCMV with with sequence homology discrepancies
• Primate model supports observation to rate of vertical CMV transmission in seronegative immunocompetent rhesus dams
• Findings underscore need to research immune responses to fully prevent infection
• In immunocompromised dams, pre-existing antibodies can protect suggesting that vaccinations to elicit IgG could also prevent pre-existing infections
• Innate immunity, T cell responses, and maternal-fetal interference and inflammation may influence CMV transmission