Monocyte Subsets in Healthy Adults and Sepsis Patients

Abstract

Monocytes play a key role in the development of immune response in bacterial infection, because of their phagocytic, antigen-presenting and secretory functions. There are three subpopulations of monocytes: “classical” CD14+CD16-, “intermediate” CD14+CD16+, and “nonclassical” CD14+dimCD16+. These monocyte subtypes have different phenotypes and functions. The ratio of appropriate subpopulations varies with development of the antibacterial response. The aim of the present research was to study phenotypes of the monocyte subpopulations in the patients with sepsis, and changes in the monocyte subpopulation ratio, depending on the presence of bacteria in circulating blood of the patients, as well as to estimate contribution of the monocyte subpopulations to the cytokine production. We observed 16 patients with sepsis (10 men and 6 women; mean age, 58±14 years, SOFA 9.4±2.1; a total of 44 blood samples) examined in dynamics. The control group included healthy adults (n = 23, 12 men and 11 women; mean age, 51±13 years). Laboratory studies included bacteriological cultures, determination of absolute and relative numbers of subpopulations of classical, intermediate and non-classical monocytes and their expression of HLA-DR and CD64, determination of IL-6, TNFα, IL-1β, IL-10 concentration in blood serum. Absolute number of monocytes was increased in the sepsis patients, the ratio of classical monocytes was also increased, like as relative and absolute numbers of intermediate cells. Meanwhile, the subpopulation of non-classical monocytes did not change significantly. The monocyte subpopulation ratio depended on the presence of bacteria in blood, i.e., a higher proportion of intermediate cells was observed in the samples positive for bacteria in blood cultures. The ratio of subpopulations was restored after elimination of bacteria from the circulation. The expression density of LPS receptor (CD14), IgG receptors (CD16 and CD64) was found to be increased, especially in the subpopulations of intermediate and nonclassical monocytes. In all subpopulations of monocytes, expression of HLA-DR is reduced, most notably in classical monocytes, least in intermediate cells. There was a significant increase in serum levels of IL-6, IL-1β, TNFα and IL-10 cytokines. Direct correlation between the absolute number of classical monocytes and IL-6 concentration was revealed, as well as intensity of multiple organ dysfunction. Increase in absolute amount of classical monocytes and IL-6 concentration might serve as an indirect criterion for evaluation of endothelial activation, an active producer of IL-6 and myeloid cell growth factors. A direct correlation between the percentage of CD14+CD16+ cells and IL-10 concentration in blood serum indicates to an important role of intermediate monocytes in IL-10 production. IL-10 suppresses the antigen-presenting function of intermediate cells, namely, expression of HLA-DR molecules, as suggested by inverse correlation between the IL-10 concentration and HLA-DR expression density on CD14+CD16+ cells. We have also determined an inverse correlation between the degree of multi-organ dysfunction and relative amount of HLA-DR+ monocytes. The larger was a classical monocyte subpopulation, the more noticeable was a decrease of this index. The studies in ratios of monocyte subpopulations help to understand the mechanisms of antibacterial protection in sepsis. Monitoring of classical monocyte numbers and serum concentrations of IL-6 is necessary for a comprehensive assessment of inflammatory response in sepsis. Determination of HLADR expression on monocytes allows us to evaluate the intensity of immune suppression in critically ill patients.

Participation of Monocyte Subpopulations in Progression of Experimental Endotoxemia (EE) and Systemic Inflammation

“…In 2010, the Nomenclature Committee of International Union of Immunological Societies [ 14 ] approved the conventional designations of three subpopulations of human monocytes: classical monocytes with high level of CD14 expression but not expressing CD16 (CD14++CD16-), intermediate monocytes expressing CD16 in addition to CD14 (CD14++CD16+), and nonclassical monocytes with virtually nondetectable CD14 and high expression of CD16 (CD14 + CD16++) [ 15 ]. In healthy humans, classical monocytes comprise around 85% of the total population of circulating monocytes, the intermediate ones comprise 5%, and the nonclassical ones are the remaining 10% [ 10 ].…”

“…Currently, there is a lack of clear understanding of the reasons for such a sharp change of count and percentage on monocyte subpopulations in blood after invasion of LPS. One can suppose that activation of cMo and iMo is determined by the need to eliminate bacteria and products of their decomposition entering the blood circulation, whereas exit from the vessels is caused by the necessity to protect tissues from the pathogen [ 8 , 10 ]. In turn, significant elevation of iMo in 24 hours points on the essential cytokine response and activation of acquired immunity to protect the host [ 35 ].…”

“…Mukherjee and coworkers observed a significant elevation of CD16+ monocyte quantity along with proportional decrease of cMo count [ 33 ]. Some authors showed that monocytosis is observed during sepsis [ 36 ], the number of classical and/or intermediate monocytes being elevated, which corresponds to the effects observed during EE [ 10 , 37 , 38 ]. ncMo did not display changes in number.…”

“…When bacterial sepsis is diagnosed, the ratio of subpopulations depends on presence of the bacteria in blood: the fraction of intermediate cells increases in cases of positive results of bacteriological cultures. After elimination of the bacteria, the ratio between subpopulations is restored [ 10 ]. On the contrary, Rodriguez-Rosales showed that the quantity of all the subpopulations of monocytes in circulation is noticeably decreased during sepsis.…”

“…Thus, EE is an example of translational research that helps to study the mechanisms of systemic inflammation and to estimate novel pharmacological interventions in humans in vivo [ 6 ]. EE can be regarded as an early step of the immune response to pathogen invasion [ 7 , 8 ], whereas the long-term changes in immune cells are studied on whole blood of patients with clinically confirmed sepsis [ 9 , 10 ].…”