The Multiplexed Opsonophagocytosis Killing Assay (MOPA) in Evaluating Specific Antibody Responses in Patients with Recurrent Infections

Assessments of antibody responses to vaccines are essential tools for evaluating humoral immune responses in subjects with suspected inborn errors of immunity (IEI). Low pneumococcal IgG levels following pneumococcal vaccination indicate suboptimal humoral immune function and are often used for diagnosis of IEI. The Multiplexed Opsonophagocytosis Assay (MOPA) measures pneumococcal antibody function (PAF) via killing of pneumococci by serum antibodies in vitro. MOPA may also be useful for clinical diagnosis but has not been studied in patients with suspected IEI. We aimed to compare PAF and Streptococcus pneumoniae IgG levels in patients with recurrent infections. We also evaluated the clinical history and other immune workup results, when available. We collected serum samples from n = 17 patients with a history of recurrent infections undergoing pneumococcal IgG measurements by their primary clinical immunologists. Baseline sera (prior to vaccine challenge) were analyzed for PAF, for the serotypes present in pneumococcal conjugate vaccine (PCV13), and S. pneumoniae 23 serotypes IgG levels using MOPA and a multiplexed bead array assay, respectively. The cohort includes subjects from 2 to 60 years of age, 53% females, 94% Caucasian, and 30% Hispanic. Preliminary results show that 76% patients had protective PAF for 85% or more of the tested serotypes. Of these, 69% patients had received a pneumococcal vaccine in the previous five years as an indication for an underlying high risk medical condition or as part of their immunization schedule. Protective results for serotypes 3, 4, 9V, and 18C were discrepant between IgG levels and PAF in more than 50% of patients. We also found discrepancy between PAF and antibody levels in 35% of patients, showing intact PAF for 5 or more serotypes despite low serotype IgG levels; 83% of whom had no significant sinopulmonary infections in the last year. MOPA analysis between pre and post pneumococcal vaccine administration, in those patients for which it is clinically indicated, is currently in process. This analysis may provide more information regarding the diagnostic performance and predictive value of MOPA in patients with suspected IEI, but preliminary data shows that MOPA might be more reflective of protection than S. pneumonia IgG levels.