Dr.Dong Chen and his colleagues recently found that most patients who have recovered from COVID-19 infection produced virus-specific antibodies and T cells, a finding which was published in the journal Immunity.

At the end of 2019, patients with COVID-19 were diagnosed in Wuhan, China, infected by SARS-CoV-2. The WHO first declared this outbreak a public health emergency of international concern and subsequently a world-wide pandemic. However, little is known about the protective immune responses induced by SARS-CoV-2. In order to understand the immune responses of COVID-19 patients, especially adaptive immunity, the team has designed and conducted this study together with their collaborators. Addressing this gap in knowledge may accelerate the development of an effective vaccine.

Blood samples from 14 mild COVID-19 cases who recently recovered from the infection were collected. Virus-encoded proteins, such as nucleocapsid protein (NP, encapsulates the viral genomic RNA) and the receptor-binding domain (RBD) of Spike protein (S-RBD, binds to receptors on host cells during viral entry) were prepared. And then different assays were carried out to study the anti-viral adaptive immunity in the patients. Compared to healthy controls, both newly discharged and follow-up patients showed production of NP as well as S-RBD-specific IgM and IgG antibodies. Taken together, these findings show that COVID-19 patients can mount antibody responses to SARS-CoV-2 proteins, and these antibodies are maintained for at least two weeks after discharge.

In addition, 13 out of 14 patients developed neutralizing antibodies, and five newly discharged patients had higher neutralizing antibody titers (ID50>500) that bind to a pseudo-virus expressing S protein. Moreover, five follow-up patients had detectable neutralizing antibodies against the pseudo-virus. As expected, the neutralizing antibody titers were positively correlated with S-RBD-specific IgG antibodies, but not with those that bind to the NP, indicating that anti-S-RBD IgG antibodies may predict neutralizing antibodies to SARS-CoV-2.

In assays for virus-specific T cell responses, five newly discharged patients had higher numbers of IFNγ-secreting T cells specific to the NP. These are the same patients who had higher titers of neutralizing antibodies. Meanwhile, seven newly discharged patients showed detectable amounts of IFNγ-secreting T cells in response to the S-RBD. By contrast, only one follow-up patient had a high number of IFNγ-secreting T cells specific to the NP, the main protease and S-RBD. Of interest, the neutralizing antibody titers were positively correlated with the number of NP-specific IFNγ-secreting T cells.

“Our findings demonstrate that most convalescent individuals show adaptive immunity to SARS-CoV-2,” says Dr. Dong. “Our work has provided a basis for further analysis of protective immunity to SARS-CoV-2, and understanding the mechanism underlying the development of COVID-19, especially in the severe cases. But it is important to note that our findings need further confirmation in a large cohort of COVID-19 patients.”

Writer: Ni Ling
Editor: Guo Lili, John Olbrich