Åke Engström

Insect immunity. Attacins, a family of antibacterial proteins from Hyalophora cecropia

Six closely related antibacterial proteins, attacins A‐F, were isolated from the hemolymph of immunized pupae of the Cecropia moth, Hyalophora cecropia. Chromatofocusing separated attacins A‐F, with isoelectric points between 5.7 and 8.3. Immunological experiments show that the attacins constitute antibacterially active forms of the previously isolated inducible immune protein P5. Their mol. wts., 20‐23 K, are similar to that of protein P5, but significantly lower than 28 K found for preP5 synthesized in vitro (see accompanying paper). The six attacins can be divided into two groups according to their amino acid composition and amino‐terminal sequences, attacins A‐D constitute a basic group and attacins E and F an acidic one. Within each group the forms are very similar. The attacins efficiently killed Escherichia coli and two other Gram‐negative bacteria isolated from the gut of a silk worm but they did not act on other Gram‐positive and Gram‐negative bacteria tested. Only growing cells of E. coli were attacked; cells suspended in phosphate buffer were inert. Besides the cecropins and lysozyme, the attacins represent a third class of antibacterial proteins in the humoral immune system of H. cecropia.

Identification of two juxtamembrane autophosphorylation sites in the PDGF beta-receptor; involvement in the interaction with Src family tyrosine kinases.

Two novel sites of autophosphorylation were localized to the juxtamembrane segment of the human platelet‐derived growth factor (PDGF) beta‐receptor. To evaluate the importance of these phosphorylation sites, receptor mutants were made in which Tyr579, Tyr581 or both were replaced with phenylalanine residues; the receptor mutants were stably expressed in porcine aortic endothelial cells. Compared with the wild‐type receptor, the Y579F and Y581F mutants were less able to mediate association with and activation of the Src family tyrosine kinases. The ability of these phosphorylation sites to mediate directly the binding of the Src family proteins was also demonstrated by using phosphotyrosine‐containing synthetic peptides representing the juxtamembrane sequence of the receptor. Both the Y579F and Y581F mutants were similar to the wild‐type receptor with regard to their protein tyrosine kinase activity and ability to induce mitogenicity in response to PDGF‐BB. A conclusive evaluation of the role of the Src family members in signal transduction could, however, not be made since our attempt to prevent completely the association by mutation of both Tyr579 and Tyr581, resulted in loss of kinase activity and was therefore not informative. The present data, together with previous observations, demonstrate a high degree of specificity in the interaction between different autophosphorylation sites in the PDGF beta‐receptor and downstream components in the signal transduction pathway.

Humoral immune response to citrullinated collagen type II determinants in early rheumatoid arthritis

Collagen type II (CII) is a relevant joint‐specific autoantigen in the pathogenesis of rheumatoid arthritis (RA). Whereas the reasons for the breakage of self tolerance to this major cartilage component are still enigmatic, T cell responses to glycosylated CII determinants in RA patients indicate that post‐translational modifications play a role. Since the conversion of arginine into citrulline by peptidylarginine deiminases (PAD) in some non‐joint‐specific antigens such as filaggrin or fibrin has been shown to give rise to RA‐specific humoral immune responses, we investigated whether PAD modification of cartilage‐specific CII might affect its recognition by circulating autoantibodies in early RA. In vitro treatment with purified PAD led to arginine deimination of native CII or of synthetic CII peptides as evidenced by amino acid analysis. The citrullination resulted in modified recognition of the immunodominant CII epitope C1III (amino acid residues 359–369) by murine and human antibodies. In a cohort of early RA patients (n=286), IgG antibodies directed toward a synthetic citrullinated C1III peptide (citC1III‐P) were detectable with a prevalence of 40.4%. The partial autoantibody cross‐reactivity between citC1III‐P and citrullinated peptides mimicking epitopes of the cytoskeletal autoantigen filaggrin suggests that autoimmunity to cartilage‐specific modified self might be a critical intermediate bridging recognition of PAD‐modified extra‐articular autoantigens with the disruption of tolerance to native cartilage constituents.

Characterization of a potassium channel toxin from the Caribbean sea anemone Stichodactyla helianthus

A peptide toxin, ShK, that blocks voltage-dependent potassium channels was isolated from the whole body extract of the Caribbean sea anemone Stichodactyla helianthus. It competes with dendrotoxin I and alpha-dendrotoxin for binding to synaptosomal membranes of rat brain, facilities acetylcholine release at an avian neuromuscular junction and suppresses K+ currents in rat dorsal root ganglion neurones in culture. Its amino acid sequence is R1SCIDTIPKS10RCTAFQCKHS20MKYRLSFCRK30TCGTC35. There is no homology with other K+ channel-blocking peptides, except for BgK from the sea anemone Bunodosoma granulifera. ShK and BgK appear to be in a different structural class from other toxins affecting K+ channels.

Antibodies to several citrullinated antigens are enriched in the joints of rheumatoid arthritis patients.

OBJECTIVE High titers of specific anti-citrullinated protein antibodies (ACPAs) are frequently present in the serum of rheumatoid arthritis (RA) patients, but their presence in synovial fluid is less well characterized. The purpose of this study was to compare the levels of antibody to 4 well-defined citrullinated candidate RA autoantigens in serum and synovial fluid and to determine whether antibodies to one citrullinated antigen are dominant over another. Furthermore, we studied their relationships with mutated citrullinated vimentin (MCV), a newly identified RA-specific serum assay, and the classic cyclic citrullinated peptide (CCP) in the synovial fluid of well-defined HLA-DR groups. METHODS Paired serum and synovial fluid samples from 290 RA patients and serum samples from 100 age- and sex-matched healthy controls were analyzed for the presence of anti-MCV and anti-CCP antibodies and for reactivity to citrullinated fibrinogen, alpha-enolase, type II collagen, and vimentin. A total of 219 of the 290 patients were genotyped for the HLA-DR shared epitope alleles. RESULTS Significantly higher proportions of antibodies against all RA-associated citrullinated antigens were found in synovial fluid as compared with serum. This was also true for the MCV and CCP responses but not for non-RA-associated anti-tetanus toxoid antibodies. As expected, we found a high correlation between citrullinated vimentin and MCV responses. All synovial fluid ACPAs were predominantly associated with HLA-DRB1*04 alleles and were confined to the CCP+/MCV+ subset of patients. CONCLUSION MCV and CCP positivity represent a similar subset of RA patients, whereas ACPAs with different fine specificities fall into subgroups of anti-CCP+/anti-MCV+ patients. The levels of all specific ACPAs were elevated in synovial fluid, suggesting that there is local antibody production and/or retention of ACPAs at the site of inflammation governed by RA-predisposing genes.

Multiple antibody reactivities to citrullinated antigens in sera from patients with rheumatoid arthritis: association with HLA-DRB1 alleles

Background: Autoantibodies to cyclic citrullinated peptides (anti-CCP) are present in most patients with rheumatoid arthritis (RA), and associate with HLA-DRB1 shared epitope (SE) alleles. Objective: To investigate reactivities of anti-CCP to various citrullinated proteins/peptides, which represent potential autoantigens in RA, and to examine the relationship between such antibodies, and their association with genetic variants within HLA-DRB1 SE alleles. Methods: Serum samples from 291 patients with established RA and 100 sex- and age-matched healthy subjects were included in this study. Sera were first analysed for presence of anti-CCP antibodies and further for IgG and IgA antibodies towards candidate autoantigens in both their native and citrullinated form including: fibrinogen, α-enolase peptide-1 and the C1-epitope of type II collagen (C1III). Antibody specificity was confirmed by cross-reactivity tests. HLA-DR genotyping was performed. Results: 72% of patients with RA were anti-CCP positive. Among the candidate autoantigens examined, IgG antibodies to citrullinated fibrinogen were found in 66% of patients’ sera and in 41% for both citrullinated α-enolase peptide-1 and citrullinated C1III. These antibodies were mainly seen in the anti-CCP-positive patient group; they were specific for their respective antigen and displayed limited cross reactivity. IgA responses were also detected, but less frequently than IgG. Anti-CCP and anti-citrullinated protein antibodies were associated with HLA-DRB1*04 rather than with HLA-DRB1*01 alleles. Conclusions: Antibodies directed against several citrullinated antigens are present in CCP-positive RA, with many patients displaying multireactivity. All specific reactivities were primarily associated with the HLA-DRB1*04 alleles, suggesting common pathways of anti-citrulline immunity.

The hemK gene in Escherichia coli encodes the N5-glutamine methyltransferase that modifies peptide release factors

Class 1 peptide release factors (RFs) in Escherichia coli are N5‐methylated on the glutamine residue of the universally conserved GGQ motif. One other protein alone has been shown to contain N5‐methylglutamine: E.coli ribosomal protein L3. We identify the L3 methyltransferase as YfcB and show that it methylates ribosomes from a yfcB strain in vitro, but not RF1 or RF2. HemK, a close orthologue of YfcB, is shown to methylate RF1 and RF2 in vitro. hemK is immediately downstream of and co‐expressed with prfA. Its deletion in E.coli K12 leads to very poor growth on rich media and abolishes methylation of RF1. The activity of unmethylated RF2 from K12 strains is extremely low due to the cumulative effects of threonine at position 246, in place of alanine or serine present in all other bacterial RFs, and the lack of N5‐methylation of Gln252. Fast‐growing spontaneous revertants in hemK K12 strains contain the mutations Thr246Ala or Thr246Ser in RF2. HemK and YfcB are the first identified methyltransferases modifying glutamine, and are widely distributed in nature.

A post-translational modification in the GGQ motif of RF2 from Escherichia coli stimulates termination of translation

A post‐translational modification affecting the translation termination rate was identified in the universally conserved GGQ sequence of release factor 2 (RF2) from Escherichia coli, which is thought to mimic the CCA end of the tRNA molecule. It was shown by mass spectrometry and Edman degradation that glutamine in position 252 is N5‐methylated. Overexpression of RF2 yields protein lacking the methylation. RF2 from E.coli K12 is unique in having Thr246 near the GGQ motif, where all other sequenced bacterial class 1 RFs have alanine or serine. Sequencing the prfB gene from E.coli B and MRE600 strains showed that residue 246 is coded as alanine, in contrast to K12 RF2. Thr246 decreases RF2‐dependent termination efficiency compared with Ala246, especially for short peptidyl‐tRNAs. Methylation of Gln252 increases the termination efficiency of RF2, irrespective of the identity of the amino acid in position 246. We propose that the previously observed lethal effect of overproducing E.coli K12 RF2 arises through accumulating the defects due to lack of Gln252 methylation and Thr246 in place of alanine.

The antibacterial effect of attacins from the silk moth Hyalophora cecropia is directed against the outer membrane of Escherichia coli.

The attacins are antibacterial proteins which accumulate in the hemolymph of the giant silk moth, Hyalophora cecropia, in response to a bacterial infection. Here we show that the permeability barrier function of the outer membrane is affected shortly after addition of attacin to growing cultures of Escherichia coli. Specifically, the penetration through the outer membrane of beta‐lactam antibiotics, chicken egg white lysozyme and the detergent Triton X‐100 was found to be facilitated. The sensitivity of E. coli to cecropin B, another antibacterial protein present in the hemolymph of H. cecropia, was also found to be increased after treatment with attacin. The results suggest that the target of the attacins in E. coli is the outer membrane. Other effects of the attacins which have been observed are likely to be indirect consequences of the alteration in the properties of the outer membrane. These effects include changes in the cell shape, irregular patterns of cell division and lysis. The minimal concentration at which the attacins affected the growth of E. coli was 1 and 0.5 microM for the neutral (pI 7) and basic (pI 9) attacins, respectively, which corresponds to less than 2% of the concentration of the attacins in the hemolymph of infected pupae.

Human ADA2 belongs to a new family of growth factors with adenosine deaminase activity

Two distinct isoenzymes of ADA (adenosine deaminase), ADA1 and ADA2, have been found in humans. Inherited mutations in ADA1 result in SCID (severe combined immunodeficiency). This observation has led to extensive studies of the structure and function of this enzyme that have revealed an important role for it in lymphocyte activation. In contrast, the physiological role of ADA2 is unknown. ADA2 is found in negligible quantities in serum and may be produced by monocytes/macrophages. ADA2 activity in the serum is increased in various diseases in which monocyte/macrophage cells are activated. In the present study, we report that ADA2 is a heparin-binding protein. This allowed us to obtain a highly purified enzyme and to study its biochemistry. ADA2 was identified as a member of a new class of ADGFs (ADA-related growth factors), which is present in almost all organisms from flies to humans. Our results suggest that ADA2 may be active in sites of inflammation during hypoxia and in areas of tumour growth where the adenosine concentration is significantly elevated and the extracellular pH is acidic. Our finding that ADA2 co-purified and concentrated together with IgG in commercially available preparations offers an intriguing explanation for the observation that treatment with such preparations leads to non-specific immune-system stimulation.