Jason A. Regules

Background: Plasmodium falciparum circumsporozoite protein (CSP) is the target of multiple malaria vaccines that include only a part of the protein, such as RTS,S and R21. The monoclonal antibodies L9 and CIS43 are directed against key CSP junctional region epitopes not included in RTS,S and R21, and next-generation vaccine candidates attempt to elicit similar antibodies. Understanding the effectiveness of multiple antibody responses against CSP peptides will inform next-generation vaccines. Methods: Using serum samples collected during controlled human malaria infection experiments that evaluated vaccine efficacy, we used a peptide array inclusive of CSP genetic variants to quantify anti-CSP antibody responses in unprotected adults who received a full-length recombinant CSP vaccine (rCSP), protected adults who received RTS,S, and unprotected adults who received RTS,S. We compared the breadth and intensity of responses to CSP variants between groups. Results: Overall, rCSP recipients had lower CSP antibody responses compared with the protected RTS,S group. Compared with the unprotected groups, protected RTS,S vaccinees had higher breadth of responses to peptides in the junctional region, central repeat region, and C-terminal region. The protected RTS,S group also had higher intensity of responses to 2 C-terminal peptides, including part of Th3R. In addition, protected RTS,S recipients had higher breadth of immunoglobulin A responses to variants of the C-terminal epitope PNDPNRNV, identified as a dominant motif by motif recognition software, and to similar sequences in the junctional region. Conclusions: Protection-associated antibody responses to the junctional region not contained in RTS,S suggest a cross-reactive, vaccine-induced response that provides additional benefit beyond antibodies targeting vaccine peptides.

Serological assessment of SARS-CoV-2 specific responses are an essential tool for determining the prevalence of past SARS-CoV-2 infections in the population especially when testing occurs after symptoms have developed and limited contact tracing is in place. The goal of our study was to test a new 10-plex electro-chemiluminescence-based assay to measure IgM and IgG responses to the spike proteins from multiple human coronaviruses including SARS-CoV-2, assess the epitope specificity of the SARS-CoV-2 antibody response against full-length spike protein, receptor-binding domain and N-terminal domain of the spike protein, and the nucleocapsid protein. We carried out the assay on samples collected from three sample groups: subjects diagnosed with COVID-19 from the U.S. Army hospital at Camp Humphreys in Pyeongtaek, South Korea; healthcare administrators from the same hospital but with no reported diagnosis of COVID-19; and pre-pandemic samples. We found that the new CoV-specific multiplex assay was highly sensitive allowing plasma samples to be diluted 1:30,000 with a robust signal. The reactivity of IgG responses to SARS-CoV-2 nucleocapsid protein and IgM responses to SARS-CoV-2 spike protein could distinguish COVID-19 samples from non-COVID-19 and pre-pandemic samples. The data from the three sample groups also revealed a unique pattern of cross-reactivity between SARS-CoV-2 and SARS-CoV-1, MERS-CoV, and seasonal coronaviruses HKU1 and OC43. Our findings show that the CoV-2 IgM response is highly specific while the CoV-2 IgG response is more cross-reactive across a range of human CoVs and also showed that IgM and IgG responses show distinct patterns of epitope specificity. In summary, this multiplex assay was able to distinguish samples by COVID-19 status and characterize distinct trends in terms of cross-reactivity and fine-specificity in antibody responses, underscoring its potential value in diagnostic or serosurveillance efforts.

The RTS,S/AS01 vaccine provides partial protection against Plasmodium falciparum infection but determinants of protection and/or disease are unclear. Previously, anti-circumsporozoite protein (CSP) antibody titers and blood RNA signatures were associated with RTS,S/AS01 efficacy against controlled human malaria infection (CHMI). By analyzing host blood transcriptomes from five RTS,S vaccination CHMI studies, we demonstrate that the transcript ratio MX2/GPR183, measured 1 day after third immunization, discriminates protected from non-protected individuals. This ratiometric signature provides information that is complementary to anti-CSP titer levels for identifying RTS,S/AS01 immunized people who developed protective immunity and suggests a role for interferon and oxysterol signaling in the RTS,S mode of action.