Analysis of СD4<sup>+</sup>CD8<sup>+</sup> double-positive T cell subpopulations in the patients with immunodeficiencies

Until recently, the studies of pathogenetic mechanisms of different diseases were focused on evaluation of the major cell populations, changes in their quantitative and qualitative characteristics, such as gene expression profile, secreted factors, surface markers, etc. Nowadays, the minor cell subpopulations, their low amounts in peripheral blood are of considerable interest. These cells may function in variable ways, by polarizing the immune response, or participating in regulation of the immune reactions. One of such minor subpopulations is presented by CD4+CD8+ double-positive T lymphocytes which comprise up to 3%, of blood lymphocyte contents. However, these cells are a predominant subpopulation among all T cells in thymus (about 75%). Peripheral double-positive T lymphocytes are a special subpopulation of T cells, because they have no markers characteristic of immature T cells of the thymus, but they bear memory cells antigens, e.g., the CD1a molecule, and a reduced amount of TREC, thus indicating a differentiated mature phenotype. At the same time, these cells may have different phenotypes and functions performed by them. However, the role of CD4+CD8+ double-positive T lymphocytes have not been well studied. There are positive correlations between an increase in double-positive T lymphocytes and demographic and anthropomorphological characteristics, such as age, gender, body mass index. It is known that the contents of this cell subpopulation increase in various pathological conditions, for example, in mononucleosis caused by the Epstein-Barr virus, cytomegalovirus, human immunodeficiency virus, hepatitis C, hemorrhagic fever virus, like as in oncological, autoimmune and allergic diseases. Thus, the peripheral CD4+CD8+T cells are assumed to play an important role in regulating immune responses under pathological conditions. We assessed the relative number of CD4+CD8+ double-positive T lymphocytes in immunodeficiencies in different groups depending on age, gender, and certain diagnostic markers, i.e., detection of Staphylococcus aureus, vitamin D deficiency, intestinal or mixed giardiasis, a positive PCR result for a viral infection, and the status of a COVID-19 reconvalescence. We obtained a statistically significant increase of of CD4+CD8+T cell numbers in both men and women, and the number of cells was also changed depending on age, but the selected criteria did not reflect the numerical changes of these cells. A more detailed study on the role of double-positive T lymphocyte subpopulations is necessary, both in immune disorders, and under normal health conditions, because CD4+CD8+ double-positive T lymphocytes can be a significant tool of immune response regulation and sustainability of immunity.