Cryoglobulins (CG) are immunoglobulins which precipitate at low temperature. The analysis of IgG subclass composition of CG is poorly reported. The aim of this study was to determine the subclasses of IgG-containing type I and mixed type II and III CG in relation to clinical manifestations.
Out of a previous series of 1675 patients, inclusion criteria were a cryoprecipitate > 1 mL and a total IgG > 300 mg/L. IgG subclasses were quantified by immunoturbidimetry, rheumatoid factor (RF), and C4 by immunonephelometry. Clinical parameters were collected from hospital charts.
CG samples from 86 patients were included, 10 type I CG and 76 mixed CG. Type I CG subclasses were IgG1 (6/10) and IgG2/IgG3 (4/10), never IgG4. IgG subclass in type II vs. III CG were 73.3 ± 15.2% vs. 52.5 ± 20.7% for IgG1 (p < 0.0001), 15.4 ± 8.2% vs. 25.9 ± 14% for IgG2 (p < 0.0001), 8.4 ± 12.4 vs. 21.2 ± 14% for IgG3 (p < 0.0001), and 3 ± 5.2% vs. 0.5 ± 1.2 for IgG4 (p < 0.0001). In mixed CG, the higher proportion of IgG4 was associated with RF positive CG (p = 0.01) and low C4 (p = 0.01). In type I CG, IgG1 were associated with severe vasculitis manifestations, IgG2/IgG3 with cutaneous or renal manifestations. In mixed CG, IgG2 was the only subclass associated with CG manifestations, with a higher concentration in asymptomatic (162.6 ± 29.5 mg/L) vs. symptomatic patients with cutaneous (103 ± 17.8 mg/L, p = 0.04) and neurological (108 ± 24 mg/L, p = 0.04) manifestations.
In type I IgG CG, IgG1 was the main CG subclass associated with CG vasculitis. In mixed CG, low IgG2 concentration was linked to CG cutaneous and neurological manifestations.