Norovirus-specific immunoglobulin A in breast milk for protection against norovirus-associated diarrhea among infants

Background: Norovirus (NV) causes acute gastroenteritis in infants. Humoral and fecal immunoglobulin A (IgA) responses have been correlated with protection against NV; however, the role of breast milk IgA against NV infection and associated diarrhea is still unknown. This study aimed to evaluate the protective role of NV-specific IgA in breast milk. Methods: We analyzed 95 breast milk samples collected from mothers enrolled in a birth cohort study conducted in Peru during 2016–2017. Samples were collected before the first NV infection of infants and tested for total IgA and NV-specific IgA using virus-like particles (VLPs) that represented GII·4 variants and non-GII·4 genotypes. Norovirus infection in infants was assessed by testing their stool samples for NV via real-time PCR. Breast milk samples were grouped according to NV infection and diarrheal status of infants: NV positive with diarrhea (NV+D+, n=18); NV positive without diarrhea (NV+D-, n=37); and NV negative without diarrhea (NV-D-, n=40). The percent positivity and titer of NV-specific IgA (Log10) were compared among groups. Findings: Breast milk IgA had a high percent positivity against different NV VLPs, especially against GII (89–100%). Higher median titer of NV-specific IgA was observed in NV+D- when compared to the NV+D+ group against GII·4 Sydney (NV+D+:500 vs. NV+D-:100, p=0·05) and GI·1 (NV+D+:100 vs. NV+D-:50, p=0·01). Although the median titer of NV IgA against GI·9 was similar, NV+D- had higher distribution of titers when compared to NV+D+ group (NV+D-:100 [interquartile range (IQR):100–500] vs. NV+D+:100 [IQR:50–200], p=0·03). Interpretations: Breast milk IgA highly reacted to different NV VLPs. NV-specific IgA may protect infants from having NV-associated diarrhea, but not from NV infection, thus reducing NV symptoms. Funding Statement: National Institute of Allergy and Infectious Diseases, National Institute of Health: 1R01AI108695–01A1 and the Japan Society for the Promotion of Science (Fostering Joint International Research B): 19KK0241 Declaration of Interests: We declare that we have no conflicts of interest. Ethics Approval Statement: The mothers included in this study were selected from a larger birth cohort study carried out in Peru during 2016-2017. The birth cohort study was approved by the Institutional Review Boards of Asociacion Benefica PRISMA, Universidad Peruana Cayetano Heredia, Johns Hopkins University, University of California-San Francisco. Informed consent were obtained from the mothers or caretakers of the infants. The analysis of archived specimen was approved by the ethics committee of the Tohoku University, Graduate School of Medicine.