COVID-SErology in Rheumatoid Arthritis (COVID-19)

Rheumatoid arthritis (RA) is the most common inflammatory rheumatism, affecting 0.5% to 1% of
the European population. Characterized by symmetrical and distal erosive polyarthritis, its
diagnosis is based on the association of the clinical assessment combined with the presence
of a chronic biological inflammatory syndrome and of citrullinated cyclic antipeptide
antibodies (CCP) and / or of rheumatoid factor. From a prognostic point of view, some factors
allow the risk prediction of the progression to structural damage (bone erosion), such as the
positivity of the rheumatoid factor or anti-CCP antibodies, and require the rapid
introduction of basic immunosuppressive therapy or biotherapy. In the context of the
SARS-CoV-2 pandemic, responsible for the COVID-19 disease, the interest of a serological test
is essential from an epidemiological point of view in order to follow the evolution of the
pandemic, and to adapt the strategy of de-confinement of the general population or of
patients at risk. However, RA patients receiving biotherapy present an increased risk of
infection. In this context of de-confinement, a SARS-CoV-2 serological analysis, before
initiating RA patients future treatment or in order to adapt their current treatment, would
be useful.

Detection of antibodies directed against SARS-CoV-2 by various detection kits is currently
under evaluation. To the lack of serological test kits on the French market is added the
complexity of verifying the sensitivity and specificity of these tests. On the other hand,
the immune response to SARS-CoV-2 is complex and still quite unknown. The concomitant
presence of several isotypes IgA, IgM and IgG is frequently observed. Thus, the isotypic
switching of Ig does not seem to take place correctly in this disease case, which randers the
serodiagnostics even more complicated to implement. Detection of the less specific IgMs would
therefore be essential, in the case of SARS-CoV-2. Rheumatoid factor (RF) is an
immunoglobulin (Ig), more often of the M type, directed against the Fc fragment of IgG. This
antibody, although not directly pathogenic, is very frequently present in RA patients (70 to
80%), without being specific for this pathology since it can be observed in other autoimmune,
infectious or hematological diseases as well. The presence of RF IgM, all the more in high
titers, is problematic while performing many serological or immunological assays since these
high levels can be responsible for false positive results.

During previous SARS epidemics (SARS-CoV-1 or MERS-CoV), false positive results of
serological tests were observed. When detecting anti-SARS-CoV-1 IgG or IgM in patients who
had not contracted SARS, the use of an ELISA kit, with plates coated with a cell lysate of
Vero-E6 cells infected with SARS-CoV-1, had been shown to induce a low number of false
positive results in healthy controls (less than 5%). However, the number of false positive
results in patients with autoimmune diseases was much higher, and varied accordingly with the
pathology (between 10% and 58%), the largest number of false positive results being obtained
in lupus patients. This phenomenon can be explained by the use of an immune test using
unpurified cell lysates, which contain antigens against which the autoantibodies present in
the patient serum could react, thus distorting the outcome of the test. Other causes of
increase in the false positive results rate have been described when performing SARS-CoV-1
serologies, such as the presence of active neoplasia, or because of a previous infection with
another coronavirus. Indeed, other coronaviruses, such as HCoV-OC43 and HCoV-229E, display a
nucleocapsid close to the one of SARS-CoV-1 : this similarity can be the source of
cross-reactivity. The same phenomenon was observed with the latest coronavirus: it was
therefore suggested to rather use recombinant antigens of SARS-CoV-2 in immunoassays, such as
ELISA for instance Given the similarity between SARS-CoV1 and SARS-CoV2, it is likely that
the various elements causing false positive results during serological tests for SARS-CoV-1,
in particular the presence of auto-antibodies including RF, could also cause false positive
results when performing serological tests targeting SARS-CoV-2. A recent Chinese study showed
that a moderate or high rate of RF was associated with a significant rate of false positive
results when searching for anti-SARS-CoV-2 IgM with a ELISA kit, which plates were however
coated with recombinant antigen. The authors therefore proposed to include a urea
dissociation step into the ELISA protocol, which seemed to improve the specificity of the
test, without altering its sensitivity.

It thus appears essential to assess the influence of RF, for its different isotypes (IgG,
IgM, IgA), on various serological tests for SARS-CoV-2 available in France, in order to
better assess these tests specificity.

The investigators propose to evaluate and compare 3 different serological tests available for
the detection of specific SARS-CoV-2 IgG and IgM. The rate of false positive results obtained
for each type of test will be evaluated on sera originated from patients with RA who have
never been in contact with SARS-CoV-2, since these sera were withdrawn before July 2019. The
relation between the incidence of SARS-Cov-2 serological assays false positive results and RF
antibody levels could be estimated, as well as the characterization of RF.