The class I protein HLA-B27 confers susceptibility to inflammatory arthritis in humans and when overexpressed in rodents for reasons that remain unclear. We demonstrated previously that HLA-B27 heavy chains (HC) undergo endoplasmic reticulum (ER)-associated degradation. We report here that HLA-B27 HC also forms two types of aberrant disulfide-linked complexes (dimers) during the folding and assembly process that can be distinguished by conformation-sensitive antibodies W6/32 and HC10. HC10-reactive dimers form immediately after HC synthesis in the ER and constitute at least 25% of the HC pool, whereas W6/32-reactive dimers appear several hours later and represent less than 10% of the folded HC. HC10-reactive dimers accumulate in the absence of tapasin or ␤ 2 -microglobulin, whereas W6/ 32-reactive dimers are not detected. Efficient formation of W6/32-reactive dimers appears to depend on the transporter associated with antigen processing, tapasin, and ␤ 2 -microglobulin. The unpaired Cys 67 and residues at the base of the B pocket that dramatically impair HLA-B27 HC folding are critical for the formation of HC10-reactive ER dimers. Although certain other alleles also form dimers late in the assembly pathway, ER dimerization of HLA-B27 may be unique. These results demonstrate that residues comprising the HLA-B27 B pocket result in aberrant HC folding and disulfide bond formation, and thus confer unusual properties on this molecule that are unrelated to peptide selection per se, yet may be important in disease pathogenesis.

Simian virus

The rate of neurogenesis in the peripheral olfactory neuroepithelium is regulated by unknown mechanisms. The members of the insulin‐like growth factor (IGF) family can influence neuronal generation, survival and/or differentiation. Several members of this family, in particular IGF‐1, are expressed at high levels in the olfactory bulb and epithelium, where they could influence the generation and/or survival of olfactory receptor neurons (ORNs). To explore the role of IGF‐1 in the olfactory epithelium (OE), we asked which cells expressed IGF‐1 receptors (IGF‐1Rs), using olfactory cell cultures and cryostat‐cut tissue sections of neonatal (postnatal day four) and adult rat OE. An antibody specific for the a subunit of the IGF‐1R densely labeled a subset of ORNs but not other cell types in sections and cultures. These ORNs were primarily immature, as determined by double labeling with neuronal markers. The number of IGF‐1R‐labeled cells as well as the levels of IGF‐1R protein (determined by immunoprecipitation and Western blotting) decreased with age, which is consistent with normal developmental changes. To study IGF‐1 effects in the intact animal, we infused IGF‐1 and related growth factors into the noses of newborn Sprague‐Dawley rats, i.e., when the epithelium is still developing. Growth factors or carrier solution (0.9% NaCl with 0.25% bovine serum albumin to prevent nonspecific binding) were applied (10 ml) to the left nostril once per day starting shortly after birth on postnatal day 1 (P1), P2 and P3, and the animals were sacrificed on P4 by decapitation. After paraformaldehyde immersion fixation, cryostat sections of the olfactory area of the nose were immunostained for the proliferating cell nuclear antigen (PCNA). Sections were position‐matched by turbinate structure and then epithelial height and area of PCNA staining at the base of the epithelium (which represents division of primarily neuronal precursors) were measured by image analysis. Both were significantly increased by rat IGF‐1 (20 ng/ml, 2.6 nM), but not insulin (20 ng/ml, 2.6 nM) or an IGF‐1 derivative, LongR3 1GF‐1 (200 ng/ml, 22 nM), that does not bind to the IGF‐1 binding proteins (IGFBPs). Thus IGF‐1 appears to influence the rate of olfactory neurogenesis, and its actions are not modified by the IGFBPs. These data suggest an important role for IGF‐1 in the OE.