Conservation of CD1 protein expression patterns in the chicken.

The CD1 molecules are cell-surface proteins that bind and present foreign lipids and glycolipids to T cells in a manner similar to the MHC system. While the mammalian CD1 antigen presentation pathway is well characterized, little is known about CD1 in non-mammalian vertebrates. Previous studies have identified two CD1 homologues in the chicken. We developed a monoclonal antibody designated NL1-1.A1 specific for the chCD1-1 isoform and have used this to characterize CD1 expression in tissues and cells of normal adult and embryonic chickens. The chCD1-1 isoform is expressed on a high proportion of cells in the spleen and bursa. Cells in the spleen that stain for CD1 are also positive for IgM and consistent with identification of these as B cells. In the skin, chCD1-1 is expressed on cells with dendritic morphology along the dermal-epidermal boundary and in epidermal sheets consistent with chicken Langerhans cells. Staining of cells in the medullary region of the chicken thymus was also observed. The CD1 proteins in mammals traffic to intracellular compartments to acquire lipid antigens for subsequent presentation to T cells on the surface. Consistent with data from mammal CD1 proteins, chCD1-1 partially co-localized with a lysosomal marker in the myeloid cell line BM2. Taken together, these data support broad distribution of chCD1-1 in both lymphoid and non-lymphoid tissues of the chicken that is remarkably similar to the distribution of CD1 isoforms in mammals.

Cycles of chromosome instability are associated with a fragile site and are increased by defects in DNA replication and checkpoint controls in yeast.

We report here that a normal budding yeast chromosome (ChrVII) can undergo remarkable cycles of chromosome instability. The events associated with cycles of instability caused a distinctive "sectoring" of colonies on selective agar plates. We found that instability initiated at any of several sites on ChrVII, and was sharply increased by the disruption of DNA replication or by defects in checkpoint controls. We studied in detail the cycles of instability associated with one particular chromosomal site (the "403 site"). This site contained multiple tRNA genes known to stall replication forks, and when deleted, the overall frequency of sectoring was reduced. Instability of the 403 site involved multiple nonallelic recombination events that led to the formation of a monocentric translocation. This translocation remained unstable, frequently undergoing either loss or recombination events linked to the translocation junction. These results suggest a model in which instability initiates at specific chromosomal sites that stall replication forks. Forks not stabilized by checkpoint proteins break and undergo multiple rounds of nonallelic recombination to form translocations. Some translocations remain unstable because they join two "incompatible" chromosomal regions. Cycles of instability of this normal yeast chromosome may be relevant to chromosome instability of mammalian fragile sites and of chromosomes in cancer cells.