Custom IgM-CH Protein & Anti-C5aR Antibody Power Fish Vaccine Adjuvant Discovery | AtaGenix

Study Context

Disease outbreaks in aquaculture cause billions of dollars in losses annually. Vaccination is the most sustainable prevention strategy, but the limited repertoire of effective fish vaccine adjuvants remains a bottleneck. Unlike mammals, fish rely heavily on innate immunity and IgM-mediated responses, meaning adjuvants must be tailored to the teleost immune system rather than borrowed from human vaccine platforms.

A 2025 study published in Fish and Shellfish Immunology (DOI: 10.1016/j.fsi.2025.110415) investigated whether the C-terminal peptide of grass carp complement C5a (C5a-CP) could serve as a species-specific adjuvant. The hypothesis: C5a-CP would activate C5aR on IgM⁺ B cells, enhancing their phagocytic activity and bridging innate complement signaling with adaptive antibody responses. Testing this required two custom reagents that don't exist commercially — recombinant grass carp IgM-CH and anti-grass carp C5aR antibodies.

Figure 1. Study overview: C5a-CP activates C5aR on grass carp IgM⁺ B cells, enhancing phagocytic activity and vaccine-induced immune protection. Challenge trials with A. hydrophila and GCRV-II demonstrated 96% relative percent survival (RPS) in the C5a-CP-adjuvanted group.

The Challenge

Two critical reagents stood between the team and their key experiments. First, quantifying IgM binding to C5a-CP required purified grass carp IgM-CH as an ELISA standard curve reference — no commercial source exists for this fish immunoglobulin. Second, proving that C5a-CP acts through the C5aR receptor on B cells required an anti-grass carp C5aR antibody for flow cytometry and immunofluorescence — again, not commercially available for this species.

Both reagents presented expression challenges: fish immunoglobulin heavy chains tend to misfold in E. coli, and developing an antibody against a fish complement receptor requires careful epitope selection to avoid cross-reactivity with other complement components.

AtaGenix's Approach

AtaGenix delivered an integrated protein + antibody solution tailored for non-model species:

• IgM-CH Protein Expression: GST-tagged grass carp IgM-CH (GenBank: DQ417927.1) was expressed in E. coli with IPTG induction. Inclusion bodies were solubilized and refolded via gradient urea dialysis. Affinity chromatography yielded >95% purity, with protein activity confirmed by ELISA. BCA quantification ensured accurate standard curve preparation (0.015625–0.25 μmol/L range).
• Anti-C5aR Polyclonal Antibody: AtaGenix designed the immunogen based on the C5aR N-terminal peptide sequence (DYGPFDFSNESYSC), targeting an extracellular domain unique to grass carp C5aR to minimize cross-reactivity. Rabbit-derived pAbs were produced and validated by immunofluorescence and flow cytometry, confirming specific labeling of C5aR⁺ cells in grass carp peripheral blood leukocytes. Delivered within 4–6 weeks.
• Functional Validation Support: AtaGenix supported ELISA-based IgM-CH activity testing and provided technical guidance for flow cytometry gating strategies to identify C5aR⁺/IgM⁺ double-positive B cells, as well as Ca²⁺ mobilization (Fluo4-AM) and phagocytosis assays (fluorescent bead uptake).

Figure 2. QC analysis of AtaGenix-produced recombinant grass carp IgM-CH protein. SDS-PAGE confirms high purity and expected molecular weight, suitable for use as an ELISA standard curve reference in IgM binding quantification assays.

Figure 3. Validation of AtaGenix-produced anti-C5aR polyclonal antibodies. Immunofluorescence and flow cytometry confirmed specific labeling of C5aR⁺ cells in grass carp peripheral blood leukocytes, enabling identification of C5aR⁺/IgM⁺ double-positive B cell populations.

Results and Impact

Armed with AtaGenix's reagents, the research team demonstrated that C5a-CP binds C5aR on grass carp IgM⁺ B cells, triggers Ca²⁺ mobilization, and significantly enhances phagocytic activity. When formulated as a vaccine adjuvant, C5a-CP achieved a remarkable 96.03% relative percent survival (RPS) in challenge trials against Aeromonas hydrophila and grass carp reovirus (GCRV-II) — substantially outperforming unadjuvanted controls. These findings establish C5a-CP as one of the most effective species-specific fish vaccine adjuvants reported to date.

This case study is based on a published research collaboration. Results may vary depending on target protein, construct design, and project scope. All proprietary client information is subject to NDA.