IgM and IgA anti-erythrocyte autoantibodies induce anemia in a mouse model through multivalency-dependent hemagglutination but not through complement activation (1)
Polymeric forms of 34-3C IgM and IgA anti-RBC autoantibody induce anemia as a result of multivalency-dependent hemaggultination and subsequent sequestration of RBC in the spleen, even in C3-deficient mice, lacking complement activation.
Different monoclonal autoantibodies derived from NZB mice showed that the IgG Fc region plays a critical role in the pathogenicity of anti-RBC autoantibodies by activating IgG Fc receptor (FcGR)-bearing effector cells as well as the complement cascade. In contrast to IgG anti- RBC autoantibodies, anti-RBC IgM autoantibodies induced anemia in association with agglutination of RBC in the spleen and liver but was unable to trigger FcR-mediated erythrophagocytosis, although the IgM isotype can activate complement efficiently.
On the other hand, 90% of serum IgA is monomeric in humans but dimeric in mice. However, pathogen specific serum IgA by vaccination or natural infection are a polymeric form in humans.
Spontaneous production of IgA anti-RBC autoantibodies were observed in NZB mice. IgM and IgA class-switch variants of the 34-3C IgG2a anti-RBC mAb derived from NZB mice.
Polymeric forms of both IgM and IgA isotypes induce anemia by agglutinating RBC, whereas their monomeric forms failed inducing hemagglutination and anemia. The polymeric IgM anti-RBC mAb can activate complement in vivo, but this activation does not play a role in the development of IgM anti-RBC-induced anemia. Autoimmune hemolytic anemia was induced by a single intraperitoneal or intravenous injection of anti-RBC mAb into 2- to 3-month-old mice.
Hemoglobin levels in plasma were measured with freshly prepared EDTA plasma. A hemagglutination assay was performed as follows: Fifty uL of serial dilutions of antibody solution were mixed with 50 uL of 0.1% mouse RBC suspension in 1% bovine serum albumin in phosphate buffered saline (BSA-PBS) in round-bottomed microtiter plates and incubated at room temperature for 1 hour.
34-3C IgM mAb developed anemia in wild-type (WT) B6 mice and in C3-/- B6 mice. The complement activation is not required for developing anemia. Polymeric IgA anti-RBC mAb (100 ug) induced anemia in all three strains of WT, C3-/- and FcRg-/-. In contrast, 34-3C IgG2a (100ug) induced anemia in WT mice, but failed in C3-/-mice and FcRg-/- (P < 0.05). Thus, there was no role for complements and FcRg-associated Fcg RI in the development of IgM- and IgA-mediated autoimmune hemolytic anemia in mice.
The affinity of the 4C8 mAb is more than 1,000 times lower than that of the 34-3C mAb. The high pathogenic potential of 34-3C IgM variant was not exclusively correlated with its high affinity against RBC.