Diagnosis of Mycoplasma Pneumoniae Infection With Detection of Specific Antibody-secreting Cells in Community-acquired Pneumonia (CAP) Patients of the Randomised Placebo-controlled Multi-centre Effectiveness Trial of Adjunct Betamethasone Therapy (myKIDS-STEP)

Community-acquired pneumonia (CAP) is a common serious infection and a leading cause of hospitalisation in children. Knowledge about the underlying pathogen is a major unmet clinical need, particularly in CAP caused by Mycoplasma pneumoniae (Mp). Timely and reliable identification is critical for initiating effective and tailored antimicrobial treatment. However, determining the causative pathogen of childhood CAP is complicated by the low yield of blood cultures and difficulty obtaining specimens from the lower respiratory tract of children. Therefore, clinicians attempt to detect potential pathogens in upper respiratory tract (URT) specimens, knowing that children carry viruses and bacteria in their URT that may or may not be causative for the current pneumonia episode. Consequently, the interpretation of diagnostic tests performed with URT specimens is limited and may lead to unnecessary antimicrobial prescriptions.

The hurdle in differentiating infection from carriage was documented recently for Mp, a frequently reported pathogen underlying CAP in children worldwide (up to 20-40% during epidemics). Current diagnostic tests, including polymerase chain reaction (PCR) of URT specimens or serology, do not reliable differentiate between Mp infection and carriage. Mp is found in the URT in up to 56% of healthy children. These findings challenge recent epidemiological data indicating Mp as the most common bacterial cause of CAP, in up to 23% of hospitalized U.S. children aged 10-17 years. A ≥4-fold increase in IgG antibody levels is still considered the "gold standard" for diagnosing M. pneumoniae infection, but has low sensitivity when e.g. compared with IgM seroconversion and/or a 2-fold IgM increase. In fact, such a definition is also not helpful in acute clinical management, as it requires acute and convalescent sera.

Circulating antigen-specific B cell responses have been investigated in vaccine studies and demonstrated to be more rapid and shorter lived than antibody responses. After exposure, antigen-specific B cells proliferate and differentiate into antibody-secreting cells (ASCs) and memory B cells. ASCs transiently circulate in the peripheral blood in the first days after an antigen encounter. In a recent observational pilot study of children with CAP and healthy controls, we showed that the detection of Mp-specific immunoglobulin (Ig) M ASCs by enzyme-linked immunospot (ELISpot) assay re-classified 15% of PCR-positive and 12% of IgM-seropositive study participants (https://doi.org/10.1164/rccm.201904-0860LE). Thus, the measurement of specific IgM ASCs by ELISpot assay is an innovative, minimally invasive, and rapid test method that optimises diagnosis of Mp CAP in children.

In view of these promising first results, the aim of this study is to establish the diagnosis of Mp infection by the measurement of Mp-specific ASCs by ELISpot in CAP patients enroled in the randomised placebo-controlled multi-centre effectiveness trial of adjunct betamethasone therapy (KIDS-STEP study, Protocol ID: NCT03474991).