Preliminary Characterization of a Chicken Serum Hemagglutination Enhancing Factor of Complement: Evolutionary and Functional Perspectives

Functional and evolutionary analyses were performed to characterize a hemagglutination enhancement phenomenon discovered in normal chicken serum, a mechanism that may be interacting with the chicken complement system. Preliminary functional characterization of the chicken hemagglutination enhancing process indicates that it is mediated by specific binding to the constant region of chicken IgY antibody bound to antigen. Based on this specificity, as well as the heat-insensitive nature of the phenomenon, it is hypothesized that the components of serum involved in the enhanced agglutination include naturally produced IgM autoantibodies as well as a soluble Fc receptor. Recent data shows an increase in IgM levels corresponding to increases in hemagglutination enhancing activity, implicating IgM in an enhancement role. However, activity that does not correlate with IgM levels is present, indicating that another factor is involved in this process. A second protein factor is in fact found only in serum macroglobulin fractions positive for activity, and its concentration fluctuates with the level of enhancement. Evolutionary analyses demonstrate the absence of key proteins involved in the alternative pathway of complement activation in the chicken, possibly demonstrating the absence a functional pathway in the avian lineage. The evolution of complement components C3, C4, and C5 was characterized utilizing codeml from the Phylogenetic Analysis by Maximum Likelihood (PAML) suite. Analysis indicates that ancestral and modern C3 lineages are evolving similarly, while a small but significant number of codons are evolving differently in the C4 and C5 lineage, possibly providing the protein regions necessary for evolution of novel function