摘要Objective To investigate how ultraviolet B (UVB)-irradiated urocanic acid (cis-UCA) affects the immune function and possible mechanisms.Methods Skin grafts were harvested from the tails of C57B mice and placed onto full thickness beds on the flanks of BALB\C mice (across a complete MHC Class Ⅰ and Ⅱ mismatch). 0.2 ml of 24% cis-urocanic acid, 100% trans-urocanic acid, and saline was separately injected ip into recipient mice prior to skin grafting. The skin allograft survival time was observed and compared between groups.A mice model for delayed-type hypersensitivity (DTH) response to DNP6-OVA which was indicated by increasing in ear thickness was established. Each mouse in Group A and B was injected respectively with 200 μg cis-UCA and trans-UCA ip 5 hours previous to sensitization with 10 μg emulsified DNP6-OVA in the subcutaneous of hind legs. Group C was control group, in which DNP6-OVA remained at the same time. After 7 days, each ear thickness of all mice including group D was measured by micrometer. Mice in the group D were examined by increase of ear thickness caused by the physical stimulus and other factors. 2 μg DNP6-OVA were injected into each pinnae thereafter. And each ear thichness was remeasured 24 hours later.The serum IgG, C3, C4 level in the mice stimulated by E. Coli was detected by ELISA assay.The lymphocytes were cultured with 24% Cis-UCA, trans-UCA, histamine, Cis-UCA cimetidine together and histamine cimetidine together respectively. After 40 hours PHA stimulating, 200 μl lymphocytes culture mediaClinical Experiment Research Laboratory, First Affiliated Hospital, Nanjing Medical University, Nanjing 210029 Jiangsu Province, China (Wang T, Chen HQ, Ni L and Ding XJ)This research was sponsored by National Natural Science Foundation of China, No. 39270646.supernatant were collected from each group to evaluate the IL-2 active level with IL-2 depending cells "CTLL" by MTT method. 68 hours later following PHA stimulating, lymphocytes proliferation index (SI) was detected by MTT method. Meanwhile, the total cells RNA of each group were extracted by one step extraction method and quantified, then the IL-2 mRNA was detected by RT-PCR.Results Cis-UCA could significantly prolong the allograft skin survival (11.4±0.71 / 9.3±0.22 days, P<0.05) comparing with the trans-UCA and PBS. Significant immunosuppression of DTH to DNP6-OVA was found in the Cis-UCA group, the P value <0.01 (t=9.48). No same effect was found in trans-UCA and PBS group. The 24% Cis-UCA immune suppression rate was 47.9%. Also we found that Cis-UCA could significantly inhibit the IL-2 bioactivity than trans-UCA (3.13±1.52 / 10.14±6.21, P<0.05) in the mice stimulated by E. Coli. The humoral immune response could be suppressed by Cis-UCA. The serum IgG level was decreased significantly in the Cis-UCA group (t=4.03, P<0.01). The serum complement level (C3 and C4) was also decreased after injecting Cis-UCA (C3: t=2.50, P<0.05; C4: t=4.63, P<0.01). Meanwhile, the results indicated that normal T lymphocytes proliferation could be inhibited by Cis-UCA. The stimulated index (SI) of T lymphocytes could be decreased by Cis-UCA. Same effects were found in histamine group. The inhibited effects could be blocked by cimetidine (H2R blocker). The trans-UCA did not show the stimulated index inhibiting effect on T lymphocytes.The same effects of Cis-UCA upon T lymphocyte's total RNA were found. The Cis-UCA and histamine decreasing the cell total RNA could be also blocked by cimetidine. Conclusions The ultraviolet immunosuppression may be mainly mediated by Cis-UCA which could effect on the H2 receptors on T lymphocytes.