Cell surface‐located sialic acids of the capsule and the lipooligosaccharide (LOS) are both pivotal virulence factors in Neisseria meningitidis, promoting survival and dissemination of this pathogen which can cause both sepsis and meningitis. With the aid of a unique set of isogenic meningococcal mutants defective in the expression of cell surface‐located sialic acids, we have demonstrated that encapsulation hinders the primary event in the development of the disease, but the spontaneous switching of encapsulated wild‐type bacteria to a capsule‐negative phenotype promotes meningococcal adherence and invasion into mucosal epithelial cells. Genetic analysis of the capsule‐negative, invasive bacteria revealed a unique mechanism for modulation of capsule expression based on the reversible inactivation of an essential sialic acid biosynthesis gene, siaA, by insertion/excision of a naturally occurring insertion sequence element, IS1301. Inactivation of siaA regulates both capsule expression and endogenous LOS sialylation. This is the first example of an insertion sequence element‐based genetic switch mechanism in the pathogenic bacterium and is an important step in the understanding of bacterial virulence.

The insertion element IS1301 has been shown to mediate capsule phase variation in Neisseria meningitidis serogroup B by reversible insertional inactivation of the siaA gene. We have determined the target site specificity of this element by cloning and sequencing the insertion sites of 12 identical IS1301 copies found in N. meningitidis B1940. A target consensus core of 5-AYTAG-3 was identified, with the central TA being duplicated following insertion. Additional features around the target sites, including extended palindromic symmetry, stem-loop formation, and the high incidence of AT tracts, indicate that other factors, such as DNA secondary structure, are involved in target recognition. The left inverted repeat of an IS1016-like element acts as a hot spot for insertion, with one insertion element combination located upstream of the frpC gene. According to further sequence analysis, we were able to place IS1301 in the IS5 subgroup within the IS4 family of elements. A survey of 135 Neisseria strains indicated the presence of IS1301 in 27.9 to 33.3% of N. meningitidis serogroup B, C, and W135 strains and in 86.7% of serogroup Y strains. IS1301 did not occur in serogroup A strains, in Neisseria gonorrhoeae, or in apathogenic Neisseria spp.

SUMMARYInterferon (IFN)-b reduces the biological activity of multiple sclerosis (MS), a presumably T cellmediated autoimmune disease of central nervous system (CNS) myelin. Co-stimulatory molecules are necessary for full T cell activation and differential expression of co-stimulatory molecules on antigenpresenting cells is thought to influence the type of effector T cell response (Th1/Th2). In this study we investigated the effects of IFN-b on the expression of co-stimulatory molecules on lymphocytes and monocytes as a potential mechanism of action of IFN-b in MS. Peripheral blood mononuclear cells (PBMCs) were stimulated with IFN-b in vitro and expression of CD80, CD86, CD40 and HLA was examined by flow cytometry and reverse-transcription polymerase chain reaction. Whereas IFN-b had no effect on the expression of these molecules on T and B lymphocytes there was a significant increase on monocytes. Correspondingly, the expression of mRNA increased after 6-18 h. This in vitro response was also observed in untreated MS patients and patients receiving treatment with IFN-b . The increase of co-stimulatory molecules on monocytes was not mediated by interleukin (IL)-10. When IFN-bstimulated monocytes were used to stimulate autologous T cells an increased secretion of IL-13 was observed. In biopsies taken from IFN-b -induced skin reactions after subcutaneous injection increased expression of CD80 mRNA was detected, indicating that IFN-b also up-regulates this co-stimulatory molecule in vivo . These data provide the background for further studies of IFN-b -induced changes of co-stimulatory molecules in MS patients.

The distribution of the meningococcal insertion sequence IS1301 was analysed in 496 strains of different serogroups and clonal lineages of Neisseria meningitidis, and in 64 neisserial strains other than N. meningitidis. IS1301 was found in meningococci, but not in apathogenic Neisseria sp. and Neisseria gonorrhoeae. The copy numbers of IS1301 varied between 2 and 17 per genome. IS1301 positive strains were mostly found among the serogroups 29E, W135, X, and Y. Clonal lineages of serogroup A, B, and C meningococci associated with epidemic meningococcal disease were rarely positive for IS1301.