T cells and COVID Vaccine
COVID-19 vaccines have been developed and distributed rapidly since the pandemic began. Instead of utilizing humoral immunity through B cells, the vaccine has been developed to boost the cellular immune system to aid in the activation and function of T cells. A crucial component to this immune response is memory T cells which are formed after the body is exposed to a virus either through infection or vaccination (1).
The COVID-19 vaccine uses neutralizing antibody responses in an attempt to prevent infection and the activation of cellular immune responses to limit the viral progression. Due to the numerous variants that have emerged since the beginning of the pandemic, the initial goals of the vaccine may be unattainable. The neutralizing antibody responses have been failing in many cases and allowing the virus to infect the upper respiratory tract due to the low serum concentration of neutralizing antibodies in the vaccine. Although the vaccine may be inefficient in preventing infection, it has proven to prevent disease progression (1).
Memory T cells are the main factor in preventing the progression of the virus. They function by recognizing the signal proteins on class I or class II HLA molecules and prevent the virus from further replication. CD4+ and CD8+ are the two major types of T cells involved that perform different functions. CD8+ T cells, or killer T cells, kill the virally infected cells and release cytokines and signal molecules to solicit additional resources to fight the infection. CD4+ T cells, or helper T cells, play a supportive role to B cells, or some can function in the same way as CD8+ cells. Memory T cells are long lasting and are formed after vaccination or infection (1).
The two mechanisms studied for protection from viruses were the use of antibodies and T cells. Neutralizing antibodies can recognize mutated viruses and block their interactions with host cells. The COVID-19 virus has an outer shell which houses spike proteins. Neutralizing antibodies bind to these spike proteins which inhibits its ability to bind to the host cell. While this was initially an effective method to prevent infection by COVID-19, the virus has mutated and gained the ability to avoid this defense mechanism. T cells recognizes short chains of amino acids that are found in the virus in locations other than the spike proteins. Because other portions of the virus do not mutate as often as the spike proteins, memory T cells are still highly effective in recognizing the virus and preventing further infection. The different variants that have emerged since the beginning of the COVID-19 pandemic have been able to avoid the neutralizing antibodies but are still rarely able to evade T cells (1).
Studies have shown that memory T cells are the major form of protection against the COVID-19 virus. Progression of severe disease has been prevented in patients that were lacking neutralizing antibodies and still had CD8+ T cells. Although infection rates rose with the omicron variant, the death rates did not directly correlate (1).
These studies are being used to develop a more efficient vaccine. Some researchers suggest using portions of the COVID-19 virus in the vaccine in order to better develop memory T cells. Researchers are also studying the efficacy of booster vaccines and the level of impact that they have (1).
References
- Wherry, E. J. & Barouch, D. H. (2022). T cell immunity to COVID-19 vaccines. Science, 377(6608). 821-822. DOI: 10.1126/science.add2897