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Obiective To study the relationship between the burden of Pneumocystis carinii (P.cadninii) and the inflammatory reaction and biochemical markers in bronchoalveotar lavage fluids (BAL.F)in a rat model of p. carirnii pneumonia (PCP).Methods Clean grade 50 male Sprague-Dawley rats were immunosuppressed by a subcutaneous injection of 25mg cortisone acetate twice a week for 8-12 weeks; the PCP model was successfully induced in 14 rats. The inflammatory reaction and biochemical markers of the activity of lactate dehydrogenass (LDH), alknline phosphatase(ALP) end type Ⅳ collagenase(matrix metalloproteinases, MMP-2,MMP-9) as well as the values of total protein (TP) and albumin (ALB) in BALF between the mild burden group of P. caninii (involved alveoli＜25% per 100 alveoli,Group A) and the moderate to severe burden group (involved alveoli≥25% per 100 alveoli, Group B)were measured. The other six clean grade SD rata served es normal control group (Group C).Results The total white cell count in BALF=was higher in Group B [(6.8±1.7)×106/L] than in Group A [(3.8±1.2)×106/L] (P＜0.01);however, there were no differences in white cell differentiation.Assays of biochemical markers showed that ALB in BALF in Group B (0.893±0.469 g/L) was increased in companison with Group A (0.262±0.169 g/L); it was only 0.026±0.021 g/L in Group C. The contents of TP and activities of LDH were higher in Group B (TP 1.756±0.706 g/L, LDH 2580±550 U/L) than inGroup A (TP 0.784±0.553 g/L, LDH 1410±620 U/L); the values of TP and LDH were 0.063±0.020 g/L and 370±250 U/L respectively in Group C.The activity of Type IV collagenase, including MMP-2 and MMP-9, was higher in Group B than in Group A ( P＜0.01) (MMP-2: 1102±169 grey value vs 459±274 grey valce; MMP-9: 1218±257 grey value vs 449±225 grey value).There was no activity of Type IV cellagenase in BALF of Group C. No statistically significant difference was obselved in ALP between the groups B and A.Conclusions These results indicate that there is a significant correlation between the burden of P. carinii in lung tissues and the irlflammatory reaction as well as biochemical markers of the rasultant aotivity of lung injury.