Functional consequences of anti-properdin IgG
Positive for anti-properdin autoantibodies LN patients, who showed dose response reactivity, were used for functional analysis. The presence of IgGs from positive for anti-properdin patients showed very weak effects on the capability of properdin to bind C3b, C3bB and C3bBb. There was a weak increase in Properdin binding to C3b (Fig. 3A, D and G) and to pro-converatase (C3b+Factor B) (Fig. 3B and H) in patients 33 and 38 in presence of anti-properdin antibodies. These effects were weak and inconsistent among the tests and patients and hence could not be considered to affect the stabilizing function of properdin.
The functional effect of anti-properdin containing IgG on the activation of the alternative pathway in serum was measured by the release of Ba. No significant difference was detected between levels of Ba fragments in IgGs from LN patients in comparison with the IgGs from healthy volunteers (data not shown).
To explore the capacity of anti-properdin positive IgG to activate complement on dying cells, purified IgGs from positive patients and healthy volunteers were incubated with late apoptotic cells in alternative pathway favoring conditions. In two patients (P9 and P35, Fig. 3J, K) was detected increased deposition of C3b on late apoptotic cells. Deposition of C3 fragments was not observed in the other two patients, positive for anti-properdin (P33 and P38, data not shown). They were the same patients in whom anti-properdin antibodies weakly increased the binding of Properdin to C3b (Fig. 3A and G) and to pro-convertase (Fig. 3B and H). IgGs isolated from two patients, who were negative for anti-properdin, but positive for anti-C3 (P32 and P17) also increased deposition of C3b on late apoptotic cells (Fig. 3L, M).
Purified IgGs from the same patients (P9, P17, P32, P33, P35, P38) as well IgGs from healthy volunteers (К85, К3, К2 and collective К) were studied for their effect on properdin deposition on late apoptotic cells. The presence of patients or healthy donors IgG did not affect the deposition of properdin on late apoptotic cells (date not shown).