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A protein which reacts with the Cx-polysaccharide of Streptococcus pneumoniae and is inhibited by phosphorylcholine was isolated from the serum of rainbow trout by affinity chromatography. The protein, which exists in monomeric and oligomeric forms in non-immune trout serum, is very similar with regard to specificity and size to the Cx-reactive protein from rabbits. A semi-quantitative analytical method for evaluating bacterial agglutination with an electronic particle counter and size distribution analyzer was developed to compare natural and acute serum levels of trout and rabbit Cx-reactive proteins. Results indicate that the poikilotherm has much higher concentrations in normal serum. The trout serum protein can also be rapidly induced to yet higher levels by both chemical and physical stress. The implications for such a protein in the teleost's natural defense system and overall homeostasis are discussed.

1. Rainbow trout, Salmo gairdneri, produce elevated amounts of a serum acute phase (C-reactive) protein (CRP) when administered a variety of chemicals of environmental importance. 2. Compounds administered in doses which induce the cytochrome(s) P450 catalytic enzymes in trout hepatic microsomes also induce serum CRP. 3. However, an interferon-inducing virus does not induce CRP. Interferon induction by the virus is not significantly inhibited by chemicals which induce trout cytochrome(s) P450. 4. Simultaneous administration of chemicals and virus or virus alone results in depression of P450 protein production and only minor induction of CRP. 5. Thus, as with mammals, a reciprocating relationship appears to exist between the hemeprotein monooxygenase and immune systems of this freshwater teleost, and C-reactive protein appears to fit the reciprocating scheme closer to the cytochromes P450 response.

Monomeric and tetrameric IgM anti-haptin antibodies isolated from the sera of rainbow trout (S. gairdnerii) by immunoaffinity chromatography were compared both immunochemically and with regard to their functional abilities to lyse haptenated trout erythrocytes in the presence of trout complement. The two populations had similar binding affinities for hapten and apparently identical L chains, but differed in H chain peptide maps and immunoreactivity with rabbit anti-trout H chain serum. These differences could not be attributed to J-chain. The abilities of the two antibody subpopulations to activate C to lyse haptenated trout erythrocytes also differed dramatically. Such functional differences are not simply explained by the greater avidity of the tetrameric form since preliminary studies show that the monomeric form of trout IgM activates C via an alternative pathway mechanism while the tetrameric form activates both classical and alternative pathway mechanisms. Results suggest divergent evolution of antibody structures involved in the familiar effector functions (C activation, transport, etc.).