Mechanisms of protection against shigellosis in children

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PROJECT SUMMARY Shigella spp. are a major global cause of diarrhea and dysentery. Children <5 years of age living in low- and middle-income countries are the most affected. Mortality is second only to rotavirus among diarrheal pathogens, and repeated bouts of disease cause lifelong disability. The incidence of Shigella diarrhea is low during the first year of life, increases dramatically in toddlers 12- to 23-month-old, surpassing all other pathogens, and decreases once again after 24 months of age. Maternal immunity likely contributes to the early shielding of disease in young infants, while subsequent exposure establishes an adaptive immunity that reduces risk of infection from 2 years of age onwards. Much of what is known about Shigella immunity comes from studies in adults, while detailed information on elements that can prevent infection in children is lacking. The currently favored and most clinically advanced vaccine concept, a parenterally delivered O-polysaccharide-protein conjugate, has failed in young children <3 years of age in field studies. Our group has access to clinical specimens from a longitudinal cohort of mothers and infants living in Malawi (where Shigella is endemic) from the time of birth to 2 years of age, with infant surveillance for Shigella infection. We propose to interrogate with unprecedented depth the continuum of Shigella immunity in these children to define maternal antibodies (Ab) that help prevent shigellosis during the first months of life and the immune responses these children acquire post exposure that reduces the risk of infection after the 2-year-old mark. Accrual of Ab up to 5 years of age will be monitored in a separate longitudinal cohort of children from Malawi. Our preliminary data revealed strong anti-microbial immunity mediated by human Ab specific for Shigella proteins (i.e. IpaB and VirG); these Ab engage innate immune cells and have distinct functional capabilities compared to Ab against LPS. In this proposal, we will test the hypothesis that protein-specific immunity is critical to protect children against Shigella infection. In Aim 1, we will interrogate the biophysical and functional properties of systemic Ab using a Systems Serology platform as well as T- and B-cell responses to Shigella proteins in the pediatric cohorts. In Aim 2, the Systems Serology approach will be used to characterize Ab in breast milk. All immunological readouts will be compared longitudinally between infected and non-infected children to unmask correlates of protective immunity. In Aim 3, we will apply the knowledge acquired in Aims 1 and 2 and novel recombinant technology to rationally engineer protein-specific human monoclonal Ab with maximal antimicrobial function using peripheral blood cells from the mothers with the highest immunity. This application is timely, given that there are neither vaccines to prevent Shigella infection and its devastating consequences, particularly in young children, nor immune therapeutics that could overcome multi-drug resistance. The synergistic effort and unique resources of G. Alter at Ragon Institute and M. Pasetti at University of Maryland assures the success of this clinically relevant proposal, which will greatly advance the field.