[Neutrophils mediate T lymphocyte function in septic mice via the CD80/cytotoxic T lymphocyte antigen-4 signaling pathway].

OBJECTIVE To investigate the effect of neutrophils on T lymphocyte function in septic mice and the role of CD80/cytotoxic T lymphocyte antigen-4 (CTLA-4) signaling pathway in this modulated effects. METHODS (1) In vivo experiment: 6-8 weeks old male C57BL/6 mice were divided into sham operation group (Sham group, n = 20), Sham+CTLA-4 antibody treatment group (Sham+aCTLA-4 group, n = 20), cecum ligation and perforation (CLP) induced sepsis model group (CLP group, n = 30) and CLP+CTLA-4 antibody treatment group (CLP+aCTLA-4 group, n = 30) according to the random number table. CLP was used to reproduce mouse sepsis model. The mice in the Sham group were treated identically but their cecums were neither punctured nor ligated. In CTLA-4 antibody treatment groups, 50 μg CTLA-4 antibody was injected intraperitoneally 6 hours and 24 hours after the operation. Forty-eight hours after operation, 6 mice in Sham group and Sham+aCTLA-4 group, 14 mice in CLP group and CLP+aCTLA-4 group were randomly selected to detect the expression of CD69 in spleen. At the same time, spleen, bone marrow and peripheral blood were collected, and the expression of CD80 on neutrophils was detected by flow cytometry. The expression of CTLA-4 on the surface of T lymphocytes in spleen was detected by immunofluorescence and flow cytometry. The remaining mice in each group were used to observe the 96-hour survival after operation. (2) In vitro experiment 1: neutrophils were extracted from bone marrow of healthy mice and stimulated with LPS (1 mg/L) for 4, 8 and 12 hours respectively. The control group was added with the same amount of phosphate buffer saline (PBS) at each time point, and the expression of CD80 was detected at each time point. (3) In vitro experiment 2: splenic T lymphocytes of healthy mice were extracted and divided into PBS control group, LPS group (final concentration of LPS 1 mg/L), neutrophil group and neutrophil+LPS group. In the latter two groups, the co-culture model of neutrophils and T lymphocytes was established, and then the corresponding treatment was given to detect the expression of CTLA-4 on the surface of T lymphocytes. With the above four groups as controls, CTLA-4 antibody treatment groups (final concentration of CTLA-4 antibody 50 mg/L) were set up respectively. After 48 hours, the level of interleukin-2 (IL-2) in the cell supernatant was detected by enzyme linked immunosorbent assay (ELISA). RESULTS (1) Results of in vivo experiment: compared with Sham group, the expression of CD80 on neutrophils in spleen, bone marrow and peripheral blood was significantly up-regulated, while the expression of CTLA-4 on the surface of T lymphocytes was significantly increased [(9.98±0.84)% vs. (3.48±0.64)%, P < 0.05]. It suggested that neutrophils may affect T lymphocytes function through CD80/CTLA-4 pathway in sepsis. Compared with CLP group, CTLA-4 antibody could significantly improve the 96-hour cumulative survival rate of CLP mice (56.25% vs. 18.75%, P < 0.05), and increase the expression of CD69 on the surface of T lymphocytes. It suggested that CTLA-4 antibodies might increase T lymphocytes activation in sepsis and improve survival. (2) Results of in vitro experiment: with the prolongation of LPS stimulation, the expression of CD80 on neutrophils gradually increased in time-dependent manner as compared with PBS control group [4 hours: (6.35±0.40)% vs. (3.41±0.40)%, 8 hours: (8.57±0.64)% vs. (3.09±0.27)%, 12 hours: (19.83±1.06)% vs. (5.16±0.36)%, all P < 0.05]. Compared with PBS control group, the expression of CTLA-4 on CD4+/CD8+ T lymphocytes was not significantly affected by LPS stimulation alone, but CTLA-4 was increased after co-culture with neutrophils [CD4+: (4.92±0.30)% vs. (3.33±0.25)%, CD8+: (4.26±0.21)% vs. (2.53±0.66)%, both P < 0.05], and the increased trend of CTLA-4 was more obvious after co-culture with LPS-stimulated neutrophils [CD4+: (6.34±0.50)% vs. (3.33±0.25)%, CD8+: (6.21±0.41)% vs. (2.53±0.66)%, both P < 0.05]. In the PBS control group and LPS group, CTLA-4 antibody had no significant effect on IL-2 secretion of T lymphocytes. Compared with PBS control group, co-culture with neutrophils could inhibit the secretion of IL-2 by T lymphocytes (ng/L: 1 938.00±68.45 vs. 2 547.00±218.00, P < 0.05), and the inhibitory effect of neutrophils stimulated by LPS was more obvious (ng/L: 1 073.00±34.39 vs. 2 547.00±218.00, P < 0.05). CTLA-4 antibodies could partially restore IL-2 secretion. In conclusion, after promoting the expression of CTLA-4 on the surface of T lymphocytes, neutrophils might mediate the inhibition of T lymphocytes function by reducing the production of IL-2. CONCLUSIONS Neutrophils mediate T lymphocytes dysfunction in sepsis, and the CD80/CTLA-4 pathway plays an important role. The CTLA-4 antibody improves survival and T lymphocytes function in sepsis mice, which may be a new method of immunotherapy for sepsis.