Figure 4. IgM isolated from SARS-CoV-2–infected patients, but not healthy donors, binds to ACE2.

Equilibrium binding titrations (A–C) and kinetic traces (D–K) of immobilized ACE2 and IgM purified from human serum, as measured by biolayer interferometry. (A–C) Normalized responses at the indicated concentrations of purified IgM from 3 healthy donors and 5 SARS-CoV-2–infected patients are plotted (data from 3 independent experiments). Equilibrium dissociation constants (K_D) calculated by fitting to a 4-parameter logistic regression model are provided. (D–K) The indicated percentages (color-coded on the right side of K) represent 2-fold dilutions of the 8 purified IgM preparations shown in A–C. Binding constants were determined via global fitting for each IgM sample using Octet Data Analysis software (assuming a 1:1 binding model). Quantitation of the data shown in A–K is provided in Supplemental Table 2.