Postgrad Med J (1992) 68, 707- 713 © The Fellowship of Postgraduate Medicine, 1992 Postgrad Med J: first published as 10.1136/pgmj.68.803.707 on 1 September 1992. Downloaded from Review Article The antigenic significance and methods ofdetection ofthe anti-neutrophil cytoplasmic autoantibodies (ANCA) Xavier Bosch and Ronald A. Asherson' Service ofGeneral Internal Medicine, Hospital Clinic i Provincial, Barcelona, Spain and 'Division of Rheumatology and Connective Tissue Diseases, St Luke's/Roosevelt Division ofColumbia University, New York, USA Introduction Systemic vasculitis is often difficult to diagnose. Cytoplasmic immunostaining pattern of gran- The recent discovery of a new class of auto- ulocytes (c-ANCA) was reported in 1982 in eight antibodies, the anti-neutrophil cytoplasmic auto- patients with segmental necrotizing glomerulo- antibodies (ANCA), provides clinicians with a nephritis.5 Later, four further patients with vas- serological test strongly supportive ofthe diagnosis culitis, glomerulonephritis and the serum presence of the commonest forms of systemic necrotizing of c-ANCA were reported.6 It was only in 1985 vasculitis. ANCA are IgG autoantibodies specific when van der Woude et al.7 noticed that c-ANCA Protected by copyright. for different constituents of neutrophil azurophilic mainly occurred in patients with Wegener's granules and monocyte lysosomes. These anti- granulomatosis, that interest in these antibodies bodies are the first serological markers for several became intense. In addition, Falk and Jennette8 forms ofsystemic vasculitis, particularly Wegener's recognized in 1988 the clinical importance of the granulomatosis and polyarteritis nodosa, as well as perinuclear immunostaining of neutrophils (p- for the commoner types ofnecrotizing and crescen- ANCA) as well as its association with myeloperox- tic glomerulonephritis.' In addition to their recog- idase (MPO). They demonstrated that this was an nized use in the diagnosis and follow-up of these artifactual pattern ofcell fixation with ethanol and conditions, it appears that ANCA may be a found these antibodies mainly in patients with pathogenic factor inducing vascular injury in idiopathic necrotizing and crescentic glomerulo- patients with ANCA-associated disease. This sus- nephritis. picion arises from in vitro evidence that ANCA are It is now well accepted that ANCA have two capable of activating neutrophils causing the main antigenic specificities each one of them release oflytic proteases and toxic oxygen radicals.2 associating with two main clinical disorders. Thus, The existence of autoantibodies against neutro- the c-ANCA mainly represent anti-proteinase 3 http://pmj.bmj.com/ phils has been known for almost 30 years. A (PR3) antibodies which mostly identify patients granulocyte-specific antinuclear factor was re- with biopsy-proven Wegener's granulomatosis. ported in 1964.3 Eight years later, Wiik and Mun- The p-ANCA represent anti-MPO antibodies the4 described a standardized method for the which are mainly seen in patients with pauci- detection of granulocyte-specific anti-nuclear immune necrotizing and crescentic glomerulo- antibodies (GS-ANA). These investigators per- nephritis showing little or no evidence of formed an indirect immunofluorescence (IIF) extra-renal involvement. 9-" on September 28, 2021 by guest. procedure on ethanol-fixed neutrophils, which re- mains the standard technique for ANCA detection today. GS-ANA were found to be specific for Methods for the detection ofANCA neutrophil and monocyte nuclei, did not react with lymphocytes, and were mainly detected in patients Several methods have been used to detect ANCA. with rheumatic diseases, especially rheumatoid These include IIF, enzyme-linked immunosorbent arthritis and Felty's syndrome, as well as in some assay (ELISA), radioimmunoassay (RIA), West- patients with ulcerative colitis. ern blotting, dot blotting and immunoprecipita- tion. The first method to detect ANCA was IIF. As Correspondence: Xavier Bosch, M.D., Servicio de mentioned above, this procedure was originally Medicina Interna General, Unidad 1, Hospital Clinic i used for the detection of GS-ANA,12 and today is Provincial, Villarroel, 170, 08036 Barcelona, Spain. still used in the same way. At the First International Accepted: 31 March 1992 ANCA Workshop held in Copenhagen in 1988, 708 X. BOSCH & R.A. ASHERSON Postgrad Med J: first published as 10.1136/pgmj.68.803.707 on 1 September 1992. Downloaded from this method was definitively adapted as the ever, a major problem in this type of assay is that reference for future studies of ANCA.'3 Some many sera containing rheumatoid factors will react investigators have also performed HL-60 cell cul- with these monoclonal antibodies and give a false- tures for detection of ANCA by 1IF and for positive result. distinguishing these from serum autoantibodies Since it is now well known that the major other than ANCA.'4 c-ANCA antigen is PR3, this protein is being used The potential problem with the subjective inter- in many laboratories for detecting anti-PR3 pretation of the two 1IF patterns prompted the antibodies in solid-phase assays."' Likewise, after rapid development of antigen-specific, solid-phase the discovery that the chief p-ANCA antigen was assays. To assess the validity of these assays, it is MPO,8 this well-characterized protein is also being important to be aware of the nature of the antigen used in solid-phase assays for MPO-ANCA detec- preparation used as a substrate as well as the tion in many centres. These assays are highly specificity and label of the secondary antibody specific and sensitive for c-ANCA and p-ANCA, used. The substrates employed in solid-phase respectively. A small percentage of patients with assays are crude extracts of cells, extracts of p-ANCA will have anti-elastase or anti-lactoferrin granules, or sandwich techniques using mono- antibodies which may also now be detected by clonal antibodies or purified proteins. The secon- means of specific ELISAs using either purified dary antibodies commonly used are directed elastase or lactoferrin as a substrate. against human IgG. However, since the presence of The IIF procedure correlates well with the IgM ANCA early in disease has been reported,'5 solid-phase assays in that the number of positive some laboratories use anti-human Ig polyclonal samples is roughly the same, but the antibody titres antibodies as the secondary antibody. show a low correlation with the solid-phase The first solid-phase assay described was a RIA readings. A high IIF titre may thus be low in the using an acid extract of neutrophils. 16 This resulted ELISA and vice versa. The ELISAs are, as in a sensitivity of96% and a specificity of80%. The expected, more sensitive than IIF, but the IIF willProtected by copyright. low specificity was due to false-positive reactions detect a few sera having specificities other than for among systemic lupus erythematosus (SLE) the antigens used for coating in the solid-phase patients. This may be expected, since a whole assays. Normally, between 80% and 90% of sam- cellular extract is used which also contains nuclear ples which are positive by IIF will be positive in antigens. Likewise, the acid extract of whole cells ELISA, and about 90% ofsamples that are positive has also been used to develop an ELISA with the in ELISA will be positive in 11F.24 same drawbacks, that is, a high background and a The strategy used in many laboratories today is high proportion of false-positive reactions."7 How- to screen for ANCA by IIF. Due to problems with ever, the solid-phase assays were greatly improved interpretation of the IIF pattern, an ELISA is now when it was shown that the c-ANCA antigen often also needed to confirm the specificity. This is localized in the primary granules,'8 and assays were especially true when a p-ANCA pattern is seen. The developed using purified extracts of these granules p-ANCA pattern can mask a c-ANCA pattern or with a response at least 20 times better as compared could actually be a true GS-ANA pattern or with the whole cell extract.'9'20 This also resulted in perhaps a regular ANA pattern seen in patients not http://pmj.bmj.com/ a high specificity (greater than 95%), a sensitivity suffering from systemic vasculitis. At all, one may of 80-90% and there were no significant problems then check for lymphocytes, which are negative with false-positive reactions from SLE patients. with p-ANCA, and again perform the IIF with Although this method mainly detects PR3, high formaldehyde fixation. The pattern for MPO- serum concentrations of MPO or other azurophilic ANCA with formaldehyde fixation will change antigens may unfortunately also give a positive from p-ANCA to granular cytoplasmic. If the result. Another approach was taken by Ludemann pattern now remains nuclear, this probably et al.2' who used an affinity column with immuno- represents the presence of ANA or a true GS- on September 28, 2021 by guest. globulin from a patient with Wegener's granulo- ANA."24 The strategy used in our centre is first to matosis. An ELISA using this method seems to be screen for ANCA by standard IIF. When c-ANCA highly specific for c-ANCA and avoids the problem are seen, we then perform an ELISA using purified with false-positivity from other autoimmune sera. PR3. When a p-ANCA pattern is seen, an ELISA However, the antigen is difficult to purify. At the with purified MPO is then performed. When the First International ANCA Workshop, this assay results are doubtful by IIF, we perform an ELISA was found to correlate very well with the assay using purified extract of primary granules as sub- using purified granules.22 Finally, monoclonal strate (Figure 1). antibodies against the c-ANCA antigen have also been used to capture the antigen in a solid-phase assay.23 This has the advantage of selecting mono- clonal antibodies to the antigen of choice.