Homology between Klebsiella pneumoniae and human aquaporin-4: no evidence for cross-reactivity in neuromyelitis optica. A study on 114 patients

Abstract

Neuromyelitis optica (NMO) is an autoimmune disorder of the central nervous system (CNS), which predominantly affects the optic nerves and the spinal cord [1, 2]. NMO is associated with antibodies to aquaporin-4 (AQP4), the most abundant water channel in the CNS, in up to 80% of cases [3–7]. There is convincing evidence now that AQP4-Ab are directly involved in the pathogenesis of NMO and its formes frustes, longitudinally extensive myelitis (LETM) and recurrent optic neuritis (rON) [8, 9]. AQP4-Ab is thought to bind to an extracellular domain of the target protein, and loop E was suggested as a probable candidate epitope [10]. The cause of AQP4-Ab autoimmunity is still unknown, but molecular mimicry, i.e. structural similarity between epitopes contained within microbial and host proteins, leading to cross-reactivity of antibodies in the host, has been discussed [11]. We, therefore, performed a BLAST search for bacterial and viral proteins sharing significant homology with loop E of AQP4 (sequence, TGASMNP ARSFGPAVIMGNWENHW) [12, 13]. The Klebsiella pneumoniae subsp. pneumoniae (Kpsp) transmembrane protein KPN_pKPN3p05929 (NCBI reference sequence: YP_001338538.1) produced significant alignment with loop E of human AQP4 (87% positives, 79% identities, 0% gaps) (Fig. 1). In contrast, no bacterial or viral proteins were found that produced significant alignments with the extracellular loops A and C of AQP4. Interestingly, KPN_pKPN3p05929 belongs to the MIP/aquaporin family as predicted by total protein sequence similarities. Moreover, Kpsp infection is not rare, and immunity to Kpsp has been previously implicated in the aetiopathogenesis of a number of autoimmune conditions, such as ankylosing spondylitis or Crohn’s disease [14]. In addition, infection with K. pneumoniae has been reported in association with a first attack of optic neuritis or AQP4-Ab positive LETM in some patients [15, 16]. We, therefore, hypothesized that infection with K. pneumoniae may play a role in the immunopathogenesis of NMO. To explore this hypothesis, we determined frequency and titres of serum IgG, IgM, and IgA antibodies to Kpsp in 48 patients with AQP4-Ab seropositive NMO spectrum disorders (NMOSD) (NMO in 32; LETM in 10; rON in 4; sex ratio [m:f] = 1:23; median age, 44 years; range, 14–69), and in a group of 49 ageand sex-matched AQP4-Ab negative control patients with multiple sclerosis (MS) according to McDonald (RRMS in 41; SPMS in 8). All samples were stored at the Department of Neurology, University of Heidelberg, at -80 C until use, were anonymized prior to testing, and were tested in a blinded fashion. To allow assessment of Kpsp proteins under native expression conditions, we used a commercially available indirect immunofluorescence assay that employs complete Kpsp bacteriae (Euroimmun, Luebeck, Germany). Testing was performed according to the manufacturer’s instructions. We found that the frequency of S. Jarius and K. P. Wandinger contributed equally.