Memory responses to ovalbumin-immunization in Mojave desert tortoises.

We immunized three groups of Mojave desert tortoises (Gopherus agassizii): a group immunized twice, a group immunized once, and a group sham-immunized. We used the antigen, ovalbumin (OVA), with Freund's adjuvant to elicit antibody responses similar to those induced by extracellular bacteria. All tortoises have relatively high levels of B1 lymphocytes and natural antibodies (NAbs), and the goal of this study was to quantify B2 lymphocyte activity (antibody production and potential proliferation) that occurs in primary and secondary immunizations against this constitutive, first line of humoral defense. Specifically, we were testing for two types of induced, immunological memory. These included an elevated long-term increase in OVA-specific induced antibodies as well as for features of B2 memory cells, such as increased numbers of circulating OVA-specific cells, increased antibody production and avidity, and proliferation in the presence of OVA. Secondary responses were faster, but without any increases in antibody titer or avidity. Both groups had long-term elevation in antibodies. Over all three groups, we found no effect of the immunization (pre-vs-post) or the number of immunizations (0, 1, 2) on the number of OVA-stimulated B cells. We found an effect of immunization, but not number of immunizations, on the amount of antibody secreted by B lymphocytes. This suggests a high constitutive level of circulating B1 lymphocytes that can be stimulated by immunization. We did not find evidence of B2 memory lymphocytes because cells could not be stimulated to proliferate. Control animals confirmed that NAbs increased due to B1 lymphocyte priming with adjuvant.