IgM memory: long lived hapten-specific memory in the newt, Notophthalmus viridescens

Abstract

While enhanced long lived secondary humoral immune responses to thymus-dependent (TD) immunogens are known to occur in mammals, they have yet to be characterized in extant ectothermic vertebrates which do not normally generate immunoglobulin isotype diversity. Moreover, examination of memory in such a vertebrate may provide insights into the controversial issue of IgM memory in mammalia.

Trinitrophenyl (TNP) conjugated to horse erythrocytes (HRBC) and to lipopolysaccharide (LPS) have been used to study primitive long lived (5 months) memory in the newt, Notophthalmus viridescens. The ability to recall TNP response memory was tested by secondary immunization with hapten conjugates of the same or a different carrier from the one used to initiate the primary response. All responses were monitored by immunocyto-adherence of pooled sensitized spleen cells. While carrier-specific priming was necessary to initiate primary anti-TNP responses when TD carriers (RBC) were used, it was not required when the more rapid secondary responses were tested. No enhanced anti-carrier responses were found. However, carrier-specific suppression of the secondary anti-hapten response was observed.

Anamnesis which was both more rapid and intense developed only when TNP-LPS was used as the primary immunogen and anti-hapten memory was recalled with TNP-sheep erythrocytes (SRBC). Daily injections of Cyclosporin A from 1 day before reimmunization, affected the resultant primary (anti-SRBC) and secondary (anti-TNP) responses differentially. Colloidal carbon injection reduced the memory response by one-half. These results suggest that cellular regulatory controls may be involved in newt memory. However, no increase in TNP-specific antigen-binding cell affinity was found in comparisons of primary and secondary responses.

Since reimmunization with TNP-LPS failed to produce enhanced responses following TNP-LPS priming, one can conclude that a thymus-independent (TI) carrier of the hapten will stimulate the generation of hapten-specific memory cells in the newt; however, their functional differentiation depends on collaborative events initiated by a TD carrier used to present the hapten.