Heterogeneity in the granulomatous response to mycobacterial infection in patients with defined genetic mutations in the interleukin 12-dependent interferon-gamma production pathway

Patients with genetic lesions in the Type-1 cytokine/cytokine receptor pathway exhibit a selective susceptibility to severe infections with poorly pathogenic mycobacteria and non-typhi salmonella spp. These experiments of nature demonstrate that IL-12-dependent IFNgamma production is critical for granuloma formation and therefore host immunity against such pathogens. The essential role of granuloma formation for protective immunity to these organisms is emphasized by the differing granuloma forming capabilities and resultant clinical sequelae observed in these patients which seems to reflect their ability to produce or respond to IFNgamma (Fig. 9). At one pole of this spectrum, represented by the complete IFNgammaR1/2 deficient patients, there is a complete absence of mature granuloma formation, whereas with the less severe mutations (i.e. partial IFNgammaR1/2, complete IL-12p40 and complete IL-12Rbeta1 deficiency), granuloma formation is very heterogenous with wide variations in composition being observed. This suggests that in the latter individuals, who produce partial but suboptimal IFNgamma responses, other influences, including pathogen virulence and host genotype may also affect the type and scale of the cellular response elicited.

Determinant spreading and immune responses to acetylcholine receptors in myasthenia gravis

In myasthenia gravis (MG), antibodies to the muscle acetylcholine receptor (AChR) cause muscle weakness. Experimental autoimmune myasthenia gravis (EAMG) can be induced by immunisation against purified AChR; the main immunogenic region (MIR) is a conformation-dependent site that includes alpha 67-76. EAMG can also occur after immunisation against extracellular AChR sequences, but this probably involves intramolecular determinant spreading. In MG patients, thymic hyperplasia and germinal centres are found in about 50%, and thymoma in 10-15%. The heterogeneous, high affinity, IgG anti-AChR antibodies appear to be end-products of germinal centre responses, and react mainly with the MIR or a site on fetal AChR; the latter contains a gamma subunit and is mainly expressed on myoid cells in the thymic medulla. T cells cloned against recombinant AChR subunits recognise principally two naturally processed epitopes: epsilon 201-219 derived from adult AChR which is expressed in muscle, and sometimes in thymic epithelium, and alpha 146-160, common to fetal and adult AChR. Since AChR is not normally co-expressed with class II, it is unclear how CD4+ responses to AChR alpha and epsilon subunits are initiated, and how and where these spread to induce antibodies against fetal AChR. Various possibilities, including upregulation of class II on muscle/myoid cells and involvement of CD8+ responses to AChR and other muscle antigens, are discussed.

Antibodies affecting ion channel function in acquired neuromyotonia, in seropositive and seronegative myasthenia gravis, and in antibody-mediated arthrogryposis multiplex congenita

A new autoimmune disease affecting the neuromuscular junction has been defined. Acquired neuromyotonia is associated with antibodies to voltage-gated potassium channels that act, at least in part, by reducing potassium channel function with resulting neuronal hyperactivity. This condition is quite frequently associated with thymoma and, in many cases, antibodies to acetylcholine receptors are present as well as antibodies to VGKC. Improvements in techniques and the availability of cloned DNA and recombinant forms of the AChR subunits have led to new observations concerning the specificity and roles of antibodies in myasthenia gravis. The transfection of a cell line with the epsilon subunit means that we can now accurately compare antibodies reactive with adult and fetal human AChR. This may help to determine the relationship between AChR subunit expression in different tissues and the induction of antibodies that bind specifically to the two forms, as well as to clarify the role of antibodies to fetal or adult AChR in causing ocular muscle symptoms. Serum antibodies from a few mothers with obstetric histories of recurrent arthrogryposis multiplex congenita in their babies specifically inhibit the function of fetal AChR. These observations not only explain the cause of some cases of arthrogryposis multiplex congenita, but also suggest that other fetal-specific antibodies might be responsible for other fetal or neonatal conditions. An animal model has been established to enable us to investigate the role of maternal serum factors in causing such disorders. Seronegative MG has been the subject of many studies from our laboratory over the last ten years. The transience of the effects of SNMG plasmas on AChR function strongly suggests that the plasma antibodies do not bind directly to the AChR, but inhibit function by some indirect mechanism. They do not appear to act via the cAMP-dependent protein kinase pathway, and studies are in progress to investigate the involvement of other second messenger systems.

Acetylcholine receptor expression in human extraocular muscles and their susceptibility to myasthenia gravis

In myasthenia gravis (MG), extraocular muscle (EOM) weakness is often an initial and persisting symptom. It has been proposed that acetylcholine receptor (AChR) from EOM is antigenically different from AChR of other innervated muscles and that the presence of antibodies to fetal AChR expressed in EOM causes their weakness. We have (1) studied mRNA expression for each of the AChR subunits (alpha, beta, gamma, delta, and epsilon) in human muscle, including EOM, and (2) compared the binding of sera from ocular myasthenia gravis (OMG) patients with fetal (alpha2 beta gamma delta) and adult (alpha2 beta epsilon delta) human AChRs. RNase protection assays showed that expression of the AChR gamma-subunit (fetal-type) mRNA in EOM was comparable with that in other innervated muscle types. By contrast, epsilon-subunit (adult-type) mRNA was expressed at much higher levels in EOM than in other muscles studied. Moreover, some OMG sera bound specifically to adult AChR. These results do not support the contention that susceptibility of EOM in MG results from expression of fetal AChR and indicate that the inclusion of antigen from a source rich in adult AChR in the MG diagnostic assay will increase the yield of positive results in OMG patients.

Human nicotinic acetylcholine receptor alpha-subunit isoforms: origins and expression

A majority of the autoantibodies in the disease myasthenia gravis (MG) are directed against the alpha-subunit of the muscle nicotinic acetylcholine receptor (AChR). Unlike AChR alpha-subunits previously characterised from other species, the human alpha-subunit exists as two isoforms. The isoforms are generated by alternate splicing of an additional exon located between exons P3 and P4, termed P3A. The 25 amino acids encoded by the P3A exon are incorporated into the extracellular region of the alpha-subunit, and so may be relevant to the pathogenesis of MG. Genomic sequences from rhesus monkey, and from dog and cat, which are susceptible to MG, were characterised between AChR alpha-subunit exons P3 and P4. Although regions homologous to the P3A exon were identified for each of these species, analysis by RT-PCR showed that they are not expressed. At variance with a previous report, constitutive expression of mRNA encoding the human P3A+ alpha-subunit isoform was not detected in heart, kidney, liver, lung or brain. Differential expression of the two alpha-subunit isoforms was not seen during fetal muscle development or in muscle from MG patients. In all cases where mRNAs encoding the two alpha-subunit isoforms have been detected, they are present at an approximate 1:1 ratio.

Suppression of in-vitro lymphoproliferative responses in acute malaria patients can be partially reversed by indomethacin

In-vitro lymphoproliferative responses to malaria antigens are suppressed in patients with acute Plasmodium falciparum infection. Studies with other parasitic diseases have suggested that monocyte/macrophage-derived prostaglandins may be responsible for immunosuppression. Since acute malaria infection is characteristically associated with fever it is likely that prostaglandin E production will also be enhanced in these patients. In this study, indomethacin, a cyclooxygenase inhibitor which blocks the synthesis of prostaglandins, was added to the culture medium during assays of lymphoproliferative responses to malaria antigens and other soluble proteins. Responses to several antigens were enhanced in the presence of indomethacin, indicating that prostaglandins may have a generalized immunosuppressive role in malaria-infected individuals. However, responses to malaria antigens were particularly enhanced by indomethacin, suggesting that malaria-specific T-cells are especially sensitive to the effects of prostaglandin, possibly due to prior activation in vivo by circulating malaria antigens.