Abstract: Objective To observe the effects of ulinastatin on immune function of splenic CD4⁺ T lymphocytes and CD4⁺ CD25⁺ regulatory T lymphocytes (Tregs) and content of high mobility group box 1 (HMGB1) in peripheral blood of severely burned rats, and to analyze the possible mechanisms. Methods Ninety-six male SD rats were divided into sham injury group, burn group, and ulinastatin group according to the random number table, with 32 rats in each group. Rats in sham injury group were sham injured on the back by immersing in 37 ℃ warm water for 12 s. Rats in burn group and ulinastatin group were inflicted with 30% total body surface area full-thickness scald (hereinafter referred to as burn) on the back by immersing in 94 ℃ hot water for 12 s. Immediately after injury, rats in each group were intraperitoneally injected with saline (40 mL/kg), meanwhile rats in ulinastatin group were intraperitoneally injected with ulinastatin (4×10⁴ U/kg), once per 12 h, till post injury hour 72. Eight rats of each group were respectively selected on post injury day (PID) 1, 3, 5, and 7 to collect abdominal aortic blood samples. Serum content of HMGB1 was detected by enzyme-linked immunosorbent assay (ELISA). And then, rats of the 3 groups were sacrificed immediately to collect spleens and separate CD4⁺ CD25⁺ Tregs and CD4⁺ T lymphocytes. Flow cytometer was used to detect positive expression rates of cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) and forkhead-winged helix transcription factor p3 (Foxp3) in CD4⁺ CD25⁺ Tregs. Content of IL-10 in culture supernatant of CD4⁺ CD25⁺ Tregs, and content of interleukin 2 (IL-2), IL-4, and γ interferon (IFN-γ) in culture supernatant of CD4⁺ T lymphocytes was detected by ELISA. The proliferative activity of CD4⁺ T lymphocytes was determined by microplate reader. The sample number of above-mentioned experiments was 8 at each time point in each group. Data were processed with analysis of variance of factorial design and LSD test. Results (1) Compared with that in sham injury group, serum content of HMGB1 of rats in burn group was significantly increased from PID 1 to 7 (with P values below 0.01). Compared with that in burn group, serum content of HMGB1 of rats in ulinastatin group was significantly decreased from PID 1 to 7 (with P values below 0.01). (2) Compared with those in sham injury group, the positive expression rates of CTLA-4 and Foxp3 in CD4⁺ CD25⁺ Tregs and content of IL-10 in culture supernatant of CD4⁺ CD25⁺ Tregs of rats in burn group were significantly increased from PID 1 to 7 (with P values below 0.01), peaking on PID 3 [(65±10)%, (76±10)%, and (28.2±4.4) pg/mL respectively]. These 3 indexes of rats in sham injury group on PID 3 were (45±7)%, (46±7)%, and (11.2±2.3) pg/mL respectively. Compared with those in burn group, the positive expression rates of CTLA-4 and Foxp3 in CD4⁺ CD25⁺ Tregs and content of IL-10 in culture supernatant of CD4⁺ CD25⁺ Tregs of rats in ulinastatin group were significantly decreased from PID 1 to 7 (P<0.05 or P<0.01), reaching the nadir on PID 7 [(43±6)%], PID 1 [(50±8)%], and PID 7 [(12.4±3.4) pg/mL] respectively. These 3 indexes of rats in burn group on PID 7, 1, and 7 were (58±8)%, (71±9)%, and (19.7±2.8) pg/mL respectively. (3) Compared with those in sham injury group, the content of IL-2 and IFN-γ in culture supernatant of CD4⁺ T lymphocytes of rats was significantly decreased, while the content of IL-4 in culture supernatant of CD4⁺ T lymphocytes of rats was significantly increased in burn group from PID 1 to 7, with P values below 0.01. Compared with that in burn group, the content of IL-2 and IFN-γ in culture supernatant of CD4⁺ T lymphocytes of rats was significantly increased, while the content of IL-4 in culture supernatant of CD4⁺ T lymphocytes of rats was significantly decreased in ulinastatin group from PID 1 to 7, P<0.05 or P<0.01. (4) Compared with that in sham injury group, the proliferative activity of CD4⁺ T lymphocytes of rats in burn group was significantly decreased from PID 1 to 7 (with P values below 0.01). Compared with that in burn group, the proliferative activity of CD4⁺ T lymphocytes of rats in ulinastatin group was significantly increased from PID 1 to 7 (with P values below 0.01). Conclusions Ulinastatin can weaken the immunosuppressive function mediated by splenic CD4⁺ CD25⁺ Tregs in severely burned rats, and improve proliferative function and secretory function of splenic CD4⁺ T lymphocytes, which may be attributed to the inhibiting effect of ulinastatin on the release of HMGB1 in large amount.