Aside from our immune response to COVID-19, another important factor in countering the disease are cellular reactions which offer immunity in case of an infection. Two important aspects have emerged in recent case studies. The first is the production of an antibody that kills the virus at different locations of the body that the virus has colonized. The other is the cellular immunity provided by T cells. The T cell immunity could be more important than antibody production as it produces a longer immunogenic response than antibodies – which can die off in a shorter period of time, even within a few weeks.
Most of the vaccines being developed are now focused on both antibody production and the evocation of T cell response. Some findings are beginning to offer hope. This is not only for those who have been infected with SARS-CoV-2 as the T cell response can function in those who have never been exposed to the virus, the research suggests. On August 4, a research paper published in Science reported that a cross-reactive T cell response against the virus is visible in people who were never exposed to the novel coronavirus. According to the study, it could be due to previous exposure to other coronaviruses that cause common cold and which are not as infectious and deadly as the one we face at the moment.
More importantly, these cross-reactive T cells can recognize parts of the spike protein on the novel coronavirus. Notably, the spike protein is essential for the virus to enter human cells. The virus recognizes specific receptors on human cells through spike proteins and eventually enters inside it.
T cells are of two types — CD4 and CD8. The CD4 type confers long-term memory to our immune system against the pathogen which these memory cells were activated to counter. With these memory T cells alive, any subsequent attack by a particular pathogen offers our immune system the ability to recognize it and act upon it.
In the paper, the authors reported that they found a range of pre-existing memory CD4+ T cells that are cross-reactive with comparable affinity to SARS-CoV-2 and the common cold coronaviruses HCoV-OC43, HCoV-229E, HCoV-NL63, or HCoV-HKU1. “Thus, variegated T cell memory to coronaviruses that cause the common cold may underlie at least some of the extensive heterogeneity observed in COVID-19 disease,” the study said.
In a statement, Daniela Weiskopf, assistant professor at La Jolla Institute and a co-author of the study, said: “We have now proven that, in some people, pre-existing T cell memory against common cold coronaviruses can cross-recognize SARS-CoV-2, down to the exact molecular structures.”
The same laboratory reported similar T cell immunity prior to this finding as well. In May, they reported that all of the convalescent patients infected by SARS-CoV-2 had developed helper T cells while 70% of them had killer T cells. The level of T cell response, as found by them, nearly corresponded with the neutralizing antibody levels. The cross-reactive T cell response in those people who were never exposed to the novel coronavirus was also reported in their study. There are other studies too which say that between 20% to 50% of the people unexposed to the novel coronavirus displayed significant reactivity to the virus.
In the May research paper, the team talked about the possible reason of the cross-reactive T cell immune response due to previous exposure to other milder coronaviruses. This time however, they have come out with more elaborate and detailed results about the cross-reactive T cell response.
Implication for Vaccines
In most of the vaccine development endeavors, the primary target is the spike protein of the novel coronavirus. Researchers are trying to develop vaccine candidates that can elicit an immune response against the particular protein in question. In doing so, the researchers are aiming at both, antibody production and T cell immunity, for a much longer time.
Candidate vaccines which have shown promising results, namely the ones being developed at Oxford, in the US and China, all have reported trying to elicit a T cell immune response along with antibody production. This variety of immunity has acquired much attention as it can provide resistance for a longer period of time.