Abstract:
Enhancement of the immune response through affinity maturation of the antibody response is a feature of the mammalian
immune system and has important implications with respect to development of vaccination strategies. However, an absence of
germinal centres and apparent lack of somatic hypermutation of immunoglobulin V genes suggests that this phenomenon does
not occur in fish. We investigated the question of affinity maturation in rainbow trout (Oncorhynchus mykiss) by measuring
antibody-antigen binding kinetics using a BIA core biosensor. Following immunization with aT -cell dependent antigen (FITCKLH),
relative binding affinities of serum and mucosal antibodies were assessed based on their dissociation rate constants
(kdiss) Adetectable serum anti-FITC response developedby 4 weeks post-immunization,and a consistent shift to higher affinity
antibody production (i.e. a decrease in kdiss) was observed over the ensuing course of the immune response. An average kmss. of
3.5 X 10-4 ± 0.27 X 10-4 sec-I was observed during early stages of the response (4 weeks), while by 6 weeks this decreased
significantly (p < 0.05). Further reduction in kdiss. was observed, with a low of 1.2X 10-4 ± 0.06 X 10-4 sec-1 being observed
by week 12. Analysis of the anti-FITC response in skin-derived mucus revealed a similar pattern of decreasing kdiss, as the
immune response progressed. While these data clearly demonstrate a 2-3 fold increase in antibody-antigen binding during the
course of the immune response in trout, the magnitude of this increase is much less than that seen in the mammalian immune
response. This may reflect differences in the mechanisms underpinning this phenomenon in divergent species.
