Proof of long-term immunological memory in cartilaginous fishes

Abstract

Immunological memory provides long-term protection against pathogen re-infection and is the foundation for successful vaccination. We have previously shown an antigen-specific recall response in nurse sharks almost one year after primary exposure. Herein, we extend the time between prime and successful recall to >8 years, the longest period for which immunological memory has been shown in any non-mammalian vertebrate. We confirm that antigen binding is mediated by monomeric IgM and IgNAR, but not pentameric IgM, in both the primary and recall phases. Our inability to find target-binding clones in recombinant VNAR expression libraries suggests that, at least in this instance, antigen-specific memory cells comprise a small fraction of the IgNAR-positive B cells in epigonal and spleen. Further, that the few memory cells present can generate a robust antigen-specific IgNAR titer following re-stimulation. Our results continue to challenge the long-held, but erroneous, belief that the shark adaptive immune system is 'primitive' when compared to that of mammals.

Keywords: Antibody; Cartilaginous fish; IgM; IgNAR; Memory; Shark.

Figure 1:

Analysis of blood plasma taken from nurse shark ‘Orange’ before immunization (pre-bleed) and following induction of a primary response (bleed 4) demonstrates (A) a robust HEL-specific IgM response and (B) low level IgNAR response by antigen-binding ELISA. Results shown are duplicate averages. Control wells were both coated and blocked with 5% MPBS.

Figure 2:

Analysis of blood plasma taken from nurse shark ‘Orange’ pre-boost and 5 weeks after boosting (post-boost) with unadjuvanted, soluble HEL, 8 years after the primary response, demonstrates robust, HEL-specific (A) IgM and (B) IgNAR memory responses by antigen-binding ELISA. Relative binding of (C) monomeric IgM and (D) pentameric IgM obtained via S300 size exclusion chromatography of pre- and post-boost plasma. Results shown are duplicate averages. Control wells were both coated and blocked with 5% MPBS.