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).