Structure of MHC class I and class II cDNAs and possible immunodeficiency linked to class II expression in the Mexican axolotl.

Despite the fact that the axolotl (Ambystoma spp. a urodele amphibian) displays a large T-cell repertoire and a reasonable B-cell repertoire, its humoral immune response is slow (60 days), non-anamnestic, with a unique IgM class. The cytotoxic immune response is slow as well (21 days) with poor mixed lymphocyte reaction stimulation. Therefore, this amphibian can be considered as immunodeficient. The reason for this subdued immune response could be an altered antigenic presentation by major histocompatibility complex (MHC) molecules. This article summarizes our work on axolotl MHC genes. Class I genes have been characterized and the cDNA sequences show a good conservation of non-polymorphic peptide binding positions of the alpha chain as well as a high diversity of the variable amino acids positions, suggesting that axolotl class I molecules can present numerous antigenic epitopes. Moreover, class I genes are ubiquitously transcribed at the time of hatching. These class I genes also present an important polylocism and belong to the same linkage group as the class II B gene; they can be reasonably considered as classical class Ia genes. However, only one class II B gene has been characterized so far by Southern blot analysis. As in higher vertebrates, this gene is transcribed in lymphoid organs when they start to be functional. The sequence analysis shows that the peptide binding region of this class II beta chain is relatively well conserved, but most of all does not present any variability in the beta 1 domain in inbred as well as in wild axolotls, presuming a limited antigenic presentation of few antigenic epitopes. The immunodeficiency of the axolotl could then be explained by an altered class II presentation of antigenic peptides, putting into question the existence of cellular co-operation in this lower vertebrate. It will be interesting to analyze the situation in other urodele species and to determine whether our observations in axolotl represent a normal feature in urodele amphibians. But already two different models in amphibians, Xenopus and axolotl, must be considered in our search for understanding immune system and MHC evolution.