The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that first emerged in late 2019 is responsible for a pandemic of severe respiratory illness. People infected with this highly contagious virus can present with clinically inapparent, mild, or severe disease. Currently, the virus infection in individuals and at the population level is being monitored by PCR testing of symptomatic patients for the presence of viral RNA. There is an urgent need for SARS-CoV-2 serologic tests to identify all infected individuals, irrespective of clinical symptoms, to conduct surveillance and implement strategies to contain spread. As the receptor binding domain (RBD) of the spike protein is poorly conserved between SARS-CoVs and other pathogenic human coronaviruses, the RBD represents a promising antigen for detecting CoV-specific antibodies in people. Here we use a large panel of human sera (63 SARS-CoV-2 patients and 71 control subjects) and hyperimmune sera from animals exposed to zoonotic CoVs to evaluate RBD's performance as an antigen for reliable detection of SARS-CoV-2-specific antibodies. By day 9 after the onset of symptoms, the recombinant SARS-CoV-2 RBD antigen was highly sensitive (98%) and specific (100%) for antibodies induced by SARS-CoVs. We observed a strong correlation between levels of RBD binding antibodies and SARS-CoV-2 neutralizing antibodies in patients. Our results, which reveal the early kinetics of SARS-CoV-2 antibody responses, support using the RBD antigen in serological diagnostic assays and RBD-specific antibody levels as a correlate of SARS-CoV-2 neutralizing antibodies in people.

CD8+ T cells are crucial for control of a number of medically important protozoan parasites, including Trypanosoma cruzi, the agent of human Chagas disease. Yet, in contrast to the wealth of information from viral and bacterial infections, little is known about the antigen specificity or the general development of effector and memory T-cell responses in hosts infected with protozoans. In this study we report on a wide-scale screen for the dominant parasite peptides recognized by CD8+ T cells in T. cruzi–infected mice and humans. This analysis demonstrates that in both hosts the CD8+ T-cell response is highly focused on epitopes encoded by members of the large trans-sialidase family of genes. Responses to a restricted set of immunodominant peptides were especially pronounced in T. cruzi–infected mice, with more than 30% of the CD8+ T-cell response at the peak of infection specific for two major groups of trans-sialidase peptides. Experimental models also demonstrated that the dominance patterns vary depending on the infective strain of T. cruzi, suggesting that immune evasion may be occurring at a population rather than single-parasite level.

SARS-CoV-2 is the cause of an ongoing pandemic that has infected over 36 million and killed over 1 million people. Informed implementation of government public health policies depends on accurate data on SARS-CoV-2 immunity at population scale. We hypothesized that detection of SARS-CoV-2 salivary antibodies could serve as a non-invasive alternative to serological testing for monitoring of SARS-CoV-2 infection and seropositivity at population scale. We developed a multiplex SARS-CoV-2 antibody immunoassay based on Luminex technology that comprised 12 CoV antigens, mostly derived from SARS-CoV-2 nucleocapsid (N) and spike (S). Saliva and sera collected from confirmed COVID-19 cases and from the pre-COVID-19 era were tested for IgG, IgA and IgM to the antigen panel. Matched saliva and serum IgG responses (n=28) were significantly correlated. The salivary anti-N IgG response resulted in highest sensitivity (100%), exhibiting a positive response in 24/24 RT-PCR-confirmed COVID-19 cases sampled at >14 days post-symptom onset (DPSO), whereas the salivary anti-receptor binding domain (RBD) IgG response yielded 100% specificity. Temporal kinetics of IgG in saliva were consistent with those observed in blood and indicated that most individuals seroconvert around 10 DPSO. Algorithms employing a combination of the IgG response to N and S antigens result in high diagnostic accuracy (100%) as early as 10 DPSO. These results support the use of saliva-based antibody testing as a non-invasive and scalable alternative to blood-based antibody testing.