Evolution

Female sticklebacks count alleles in a strategy of sexual selection explaining MHC polymorphism. Reusch, T. B. H., Häberli, M. A., Aeschlimann, P. B. & Milinski, M. Nature 414, 300–302 (2001) [PubMed]

Several mechanisms have been suggested by which allelic diversity at the major histocompatibility complex (MHC) is maintained. Reusch and co-workers now describe a newly discovered mechanism — rather than selecting a mate with a dissimilar MHC genotype, female sticklebacks 'count' the number of MHC alleles and prefer males with greater allelic diversity as opposed to males with fewer alleles.

Tolerance

Corticotropin-releasing hormone promotes blastocyst implantation and early maternal tolerance. Makrigiannakis, A. et al. Nature Immunol. 2, 1018–1023 (2001) [PubMed]

This paper describes a new mechanism of maternal tolerance. Corticotropin-releasing hormone (CRH) is released after embryo implantation and was shown to induce Fas ligand expression by trophoblast cells, leading to apoptosis of activated T cells that express the death receptor Fas. CRH inhibition was shown to reduce successful embryo implantation in rats. These results indicate that CRH-induced expression of Fas ligand leads to the killing of activated T cells that would otherwise harm the embryo during implantation.

Allergy

Hyper immunoglobulin E response in mice with monoclonal populations of B and T lymphocytes. de Lafaille, M. A. et al. J. Exp. Med. 194, 1349–1360 (2001) [PubMed]

IgE production driven by TH2 cells is a central pathogenic mechanism in allergy. When mice with monoclonal populations of ovalbumin (OVA)-specific B cells and haemagglutinin (HA)-specific CD4+ T cells were immunized with OVA–HA, unusually high levels of IgE were produced. But this hyper IgE response was prevented by transferring normal, polyclonal CD4+/αβ+ T cells. Both CD25+ and CD25− regulatory T-cell subsets can mediate this effect, which involves inhibition of TH2 development.

Natural killer-cell activation

Activation of NK cell-mediated cytotoxicity by a SAP-independent receptor of the CD2 family. Bouchon, A., Cella, M., Grierson, H. L., Cohen, J. L. & Colonna, M. J. Immunol. 167, 5517–5521 (2001) [PubMed]

Marco Colonna's group has identified a new member of the CD2 family called CD2-like receptor activating cytotoxic cells or CRACC. Although CRACC has cytoplasmic motifs similar to those that recruit SAP (SLAM-associated protein) to other CD2 molecules, CRACC can activate natural killer cell-mediated cytotoxicity in the absence of SAP via an extracellular signal-regulated kinase-dependent signalling pathway.