Markers of experimental acute inflammation in the Wistar Han rat with particular reference to haptoglobin and C-reactive protein

Abstract

C-reactive protein (CRP), haptoglobin (Hp) and fibrinogen (Fbgn) are acute phase reactants (APRs), the blood levels of which increase during acute inflammation. However, although the levels of these APRs are used to monitor inflammation in man, their usefulness and sensitivity as markers of inflammation in rodents are less clear. We therefore wished to evaluate, in a comparative fashion, a prototype immunoassay for serum CRP, a commercial assay for serum Hp, and an automated assay for Fbgn, using a model of acute inflammation in the rat. Additionally, pro-inflammatory cytokines and serum protein fractions were also measured. The model of inflammation used was the intraperitoneal injection of Freund's complete adjuvant (FCA). In a concluding experiment, findings with Hp in the FCA rat model were validated in a toxicologically relevant study involving the induction of acute hepatic inflammation using the model hepatotoxicant carbon tetrachloride (CCl₄). Female Wistar Han rats were treated with a single injection of FCA in a dose–response study (1.25–10.0 ml/kg, sampling at 36 h) and two time-course studies (over 40 h and 21 days). In a final experiment, rats were dosed with CCl₄ at 0.8 ml/kg and sampled over a 17-day period. In FCA and CCl₄ experiments, serum/plasma was prepared and tissues taken at autopsy for histological assessment (CCl₄ study only). In the dose–response study, serum CRP, Hp and plasma Fbgn were increased at all FCA dose levels at 36 h post-dosing. Serum α₂ and β₁ globulin fractions were also increased, while albumin levels were decreased. In the 40-h time-course study, CRP levels peaked at 25–40 h post-dosing, to approximately 120% of control (as 100%). Hp levels increased to a maximum at 25 and 40 h post-dosing with values greater than 400% of control, and α₂ and β₁ globulin fractions peaked at 30 and 40 h post-dosing to 221 and 187% of control, respectively. Increased serum interleukin-6 (IL-6) and interleukin-1β (IL-1β) levels peaked at 20 h (11-fold) and 25 h (19-fold), respectively. In a 21-day time-course study, no increased CRP levels were measured despite elevated levels of Hp, which peaked at 36 h (approximately 7-fold above control), and remained elevated up to 21 days. IL-6 and IL-1β levels peaked at 12 h (19-fold) and 24 h (28-fold), respectively. Liver histopathology of animals treated with CCl₄ showed centrilobular hepatocellular degeneration and necrosis (most significant at 36 h) with an inflammatory response (most significant at 48 h). Resolution of the lesion was complete by 4 days post-dosing. Serum alanine aminotransferase, aspartate aminotransferase and glutamate dehydrogenase levels peaked at 36 h post-dosing. Hp levels increased maximally at 48 h (426% of control). We conclude that serum CRP is a poor marker of acute inflammation in the rat in comparison with serum Hp and plasma Fbgn. Between Hp and Fbgn, serum Hp is shown to be the most sensitive and useful marker of acute inflammation.