Axolotl My last post talked about various species that have limited MHC diversity in their population. I got carried away and forgot to mention the paper that actually prompted that long ramble.

Axolotls are a fairly popular laboratory animal; they are salamanders, famous for their ability to regenerate limbs and for neotony — at least, they’re famous in the limited social circles where neotony can be a claim to fame. They’re also known (in even more limited circles) to be immunodeficient: While they do have all the components of an adaptive immune system, their antibody and T cell responses are remarkably slow and ineffective:1

Indeed, urodele amphibians do not react to soluble antigens. Their humoral immune responses are mediated by one unique class, the IgM, and are not anamnestic. A cellular co-operation has not been demonstrated during this humoral immune response; on the contrary, thymectomy, X-ray irradiation or corticosteroid treatment enhances the humoral immune response. Their MLR are usually very poor, and their transplantation reactions are chronic (21 days median survival time) …

(It may be worth noting that in the limited number of amphibians that have been studied to date — i.e. Xenopus — the larval form and the adult form have rather different immune systems; since the axolotl is permanently in a “larval” state, perhaps its immune system is also “immature”? However, the Xenopus larval immune system is apparently more effective than the axolotls’, so who knows.)

At any rate, it had been suggested that axolotl immune responses were defective because they have limited MHC diversity. Aside from the graft rejection, the logic of this link is not at all clear to me. Why would lack of diversity cause a slow antibody response in a single individual? The original paper says

The result of this non-diverse antigenic presentation could be a poor T-helper stimulation (considering the numher of different T clones stimulated), resulting in an extremely low cytokine synthesis and the absence of any cellular co-operation.

–which makes little sense to me; the authors seem to have confused population diversity with individual diversity.

Blogging on Peer-Reviewed ResearchAnyway, it’s all moot, because (and here after two posts I finally reach my original point) the limited diversity seems to be mainly because the original authors sampled laboratory axolotls, which are of course mostly derived from a handful of founders. A recent paper shows that wild axolotls — even though they are a small, critically-endangered population that went through a population bottleneck — have reasonably normal diversity.2 This paper is itself based on a small number of individuals — just nine — and although the sample size makes it hard to be sure I’d guess there is less diversity than “normal” species, but it’s clearly more than was originally claimed back in 1998:

Our results clearly overturn the supposition regarding lack of DAB polymorphism in A. mexicanum. Evidence for balancing selection summarized above also casts doubt on the more general proposition that this species is immunodeficient, although the small sample size limits the strength of this conclusion.

  1. Tournefier, A., Laurens, V., Chapusot, C., Ducoroy, P., Padros, M. R., Salvadori, F. et al. (1998). Structure of MHC class I and class II cDNAs and possible immunodeficiency linked to class II expression in the Mexican axolotl. Immunol Rev, 166, 259-277.[]
  2. Richman, A.D., Herrera, G., Reynoso, V.H., Mendez, G., Zambrano, L. (2007). Evidence for balancing selection at the DAB locus in the axolotl, Ambystoma mexicanum. International Journal of Immunogenetics, 34(6), 475-478. DOI: 10.1111/j.1744-313X.2007.00721.x[]