FUNCTION OF COMPONENTS OF COMPLEMENT IN IMMUNE HEMOLYSIS

Competitive inhibition of the classical complement pathway using exogenous single-chain C1q recognition proteins

Complement component 1q (C1q) is a protein complex of the innate immune system with well-characterized binding partners that constitutes part of the classical complement pathway (CP). In addition, C1q was recently described in the central nervous system as having a role in synapse elimination both in the healthy brain and in neurodegenerative diseases. However, the molecular mechanism of C1q-associated synapse phagocytosis is still unclear. Here, we designed monomer and multimer protein constructs which comprised the globular interaction recognition parts of mouse C1q (gC1q) as single-chain molecules (sc-gC1q proteins) lacking the collagen-like effector region. These molecules, which can competitively inhibit the function of C1q, were expressed in an E. coli expression system, and their structure and capabilities to bind known CP activators were validated by mass spectrometry, analytical size exclusion chromatography, analytical ultracentrifugation, circular dichroism spectroscopy, and ELISA. We further characterized the interactions between these molecules and immunoglobulins and neuronal pentraxins using surface plasmon resonance spectroscopy. We demonstrated that sc-gC1qs potently inhibited the function of C1q. Furthermore, these sc-gC1qs competed with C1q in binding to the embryonal neuronal cell membrane. We conclude that the application of sc-gC1qs can reveal neuronal localization and functions of C1q in assays in vivo and might serve as a basis for engineering inhibitors for therapeutic purposes.

THE RELATION TO THE COURSE OF AVIAN MALARIA OF BIOTIN AND A FAT-SOLUBLE MATERIAL HAVING THE BIOLOGICAL ACTIVITIES OF BIOTIN

Biotin deficiency produced by a synthetic biotin-deficient diet was as effective in decreasing the resistance of chickens to infection with Plasmodium lophurae as biotin deficiency produced by a diet high in egg white. In moderately biotin-deficient ducks Plasmodium cathemerium at first multiplied more slowly than in adequately fed controls. The parasitemia in the deficient animals later overtook that in the controls and attained higher peak parasite numbers. The multiplication of P. cathemerium was notably inhibited in ducks inoculated when they were approaching death from biotin deficiency. The total and differential leucocyte count of biotin-deficient chickens did not differ significantly from that of adequately fed controls. During infection of chickens with P. lophurae and ducks with either P. lophurae or P. cathemerium significant changes occur in the concentration in the plasma of free biotin and of a material which on hydrolysis yields a fat-soluble substance (FSF) having the biological activities of biotin but differing chemically from it. In ducks which survived infections with P. cathemerium or P. lophurae the biotin concentration rose very early in the course of the infection, before there was any anemia. It fell slightly, rose to a peak at about the time of the peak parasite number, and then returned to normal. The concentration of bound FSF, which was determined in terms of its biotin activity, increased at first, then decreased, then rose and continued at a high level throughout the period of decline in parasitemia. In most of the animals which died of either infection the free biotin, instead of returning to normal, rose to very high values Just before death, while the bound FSF, instead of remaining at a high value, fell to very low values, reaching zero in several animals. Greater resistance seemed to be associated with a greater excess of bound FSF over free biotin. In animals about to die the free biotin exceeded the bound FSF. The biotin content of the liver of ducks and chickens fed an adequate diet and killed just after having undergone an infection with either P. lophurae or P. cathemerium was much less than that of control uninfected animals. When P. lophurae was cultured in vitro in suspensions of duck erythrocytes a very wide range in biotin concentration in the culture fluid did not affect its rate of multiplication. Plasma protein fractions relatively rich in FSF at a concentration of 0.75 per cent inhibited multiplication, while comparable concentrations of plasma fractions poor in FSF did not. The results obtained fit the assumption that the substance in plasma which yields FSF is directly concerned in resistance to avian malaria.

A COMPARISON OF HUMAN AND GUINEA PIG COMPLEMENTS AND THEIR COMPONENT FRACTIONS

1. Defects in methods previously proposed for the estimation of complement components are: failure to ensure an excess of the desired components and failure to ensure absence of anticomplementary effects in the dilution ranges used. Existing data are therefore subject to these uncertainties. 2. Methods are proposed for controlling the adequacy of the reagent for each component, for using it in dilutions below its anticomplementary range, and for reinforcing it with necessary components if these are present in inadequate amounts. 3. Titrations are given of the four components in human and guinea pig complements and in "midpiece," "endpiece," and in the various reagents used, including one with C'3 reactivity, solely, and one withC'4 reactivity. 4. C'3 is shown to be the component which usually limits the titer in guinea pig complement, and C'2 the component of lowest titer in human complement. 5. In immune hemolysis, each component of human complement may be replaced by the corresponding component of guinea pig complement and vice versa.

Studies on immune hemolysis: preparation of a stable and highly reactive complex of sensitized erythrocytes and the first component of complement (EAC'1): inactivation of cell-fixed C'1 by some complement reagents

The interaction of sensitized sheep crythrocytes (EA) with guinea pig complement inactivated by treatment with hydrazine or ether has been studied. In both cases EA is converted into EAC'1. The hemolytic reactivity of this complex with respect to a complement reagent that contains C'4, C'2, and C'3 (R1) decays rapidly in the presence of hydrazine-treated complement (R4) or decomplemented guinea pig serum, but not in the presence of ether-treated guinea pig serum or of saline. The rate of decay of EAC'1 is dependent on the temperature, the reaction volume, and the amount of the inactivating reagent. The capacity of R4 and of decomplemented serum to inactivate EAC'1 can be markedly reduced by treatment with ether. The essential conditions for obtaining an optimally reactive EAC'1 consist in exposing EA to ether-treated guinea pig serum in the cold. This procedure yields a highly reactive EAC'1, even when the cells are sensitized with suboptimal amounts of amboceptor.

The inactivation of complement and its components by plasmin

Human complement is inactivated by plasmin, the proteolytic enzyme of plasma or serum active at or near neutrality. The addition of streptokinase to human serum, which converts plasminogen to plasmin, also causes the inactivation of complement components C'2 and C'4 and varying amounts of C'1. C'3 is the most resistant to inactivation by plasmin. Chloroform-activated human plasmin and bovine plasmin also destroy these components of complement, but are less effective than the streptokinase-activated enzyme. The inactivation of complement by the addition of streptokinase to human serum is inhibited by high hydrogen ion concentrations, low temperature, and elevated ionic strength. The inactivation of the components of complement in various fractions of serum is influenced by the available plasminogen and the content of plasmin inhibitors in these fractions. Certain similarities are pointed out between the components of complement and the factors in the plasmin system and between the inactivation of the components of complement by antigen-antibody reactions, by specific agents, and by plasmin. The possible significance of these relationships in immune hemolysis and complement fixation, and the possible role of the plasmin system in the instability of complement and the development of anticomplementary properties in serum are discussed.