Immunological and physiological characteristics of the rapid immune hemolysis of neuraminidase-treated sheep red cells produced by fresh guinea pig serum

Sialylation of CD55 by ST3GAL1 Facilitates Immune Evasion in Cancer

Altered glycosylations, which are associated with expression and activities of glycosyltransferases, can dramatically affect the function of glycoproteins and modify the behavior of tumor cells. ST3GAL1 is a sialyltransferase that adds sialic acid to core 1 glycans, thereby terminating glycan chain extension. In breast carcinomas, overexpression of ST3GAL1 promotes tumorigenesis and correlates with increased tumor grade. In pursuing the role of ST3GAL1 in breast cancer using ST3GAL1-siRNA to knockdown ST3GAL1, we identified CD55 to be one of the potential target proteins of ST3GAL1. CD55 is an important complement regulatory protein, preventing cells from complement-mediated cytotoxicity. CD55 had one N-linked glycosylation site in addition to a Ser/Thr-rich domain, which was expected to be heavily O-glycosylated. Detailed analyses of N- and O-linked oligosaccharides of CD55 released from scramble or ST3GAL1 siRNA-treated breast cancer cells by tandem mass spectrometry revealed that the N-glycan profile was not affected by ST3GAL1 silencing. The O-glycan profile of CD55 demonstrated a shift in abundance to nonsialylated core 1 and monosialylated core 2 at the expense of the disialylated core 2 structure after ST3GAL1 silencing. We also demonstrated that O-linked desialylation of CD55 by ST3GAL1 silencing resulted in increased C3 deposition and complement-mediated lysis of breast cancer cells and enhanced sensitivity to antibody-dependent cell-mediated cytotoxicity. These data demonstrated that ST3GAL1-mediated O-linked sialylation of CD55 acts like an immune checkpoint molecule for cancer cells to evade immune attack and that inhibition of ST3GAL1 is a potential strategy to block CD55-mediated immune evasion.

Complement C5b-9 and Cancer: Mechanisms of Cell Damage, Cancer Counteractions, and Approaches for Intervention

The interactions of cancer cells with components of the complement system are highly complex, leading to an outcome that is either favorable or detrimental to cancer cells. Currently, we perceive only the "tip of the iceberg" of these interactions. In this review, we focus on the complement terminal C5b-9 complex, known also as the complement membrane attack complex (MAC) and discuss the complexity of its interaction with cancer cells, starting with a discussion of its proposed mode of action in mediating cell death, and continuing with a portrayal of the strategies of evasion exhibited by cancer cells, and closing with a proposal of treatment approaches targeted at evasion strategies. Upon intense complement activation and membrane insertion of sufficient C5b-9 complexes, the afflicted cells undergo regulated necrotic cell death with characteristic damage to intracellular organelles, including mitochondria, and perforation of the plasma membrane. Several pro-lytic factors have been proposed, including elevated intracellular calcium ion concentrations and activated JNK, Bid, RIPK1, RIPK3, and MLKL; however, further research is required to fully characterize the effective cell death signals activated by the C5b-9 complexes. Cancer cells over-express a multitude of protective measures which either block complement activation, thus reducing the number of membrane-inserted C5b-9 complexes, or facilitate the elimination of C5b-9 from the cell surface. Concomitantly, cancer cells activate several protective pathways that counteract the death signals. Blockage of complement activation is mediated by the complement membrane regulatory proteins CD46, CD55, and CD59 and by soluble complement regulators, by proteases that cleave complement proteins and by protein kinases, like CK2, which phosphorylate complement proteins. C5b-9 elimination and inhibition of cell death signals are mediated by caveolin and dynamin, by Hsp70 and Hsp90, by the mitochondrial stress protein mortalin, and by the protein kinases PKC and ERK. It is conceivable that various cancers and cancers at different stages of development will utilize distinct patterns of these and other MAC resistance strategies. In order to enhance the impact of antibody-based therapy on cancer, novel precise reagents that block the most effective protective strategies will have to be designed and applied as adjuvants to the therapeutic antibodies.

Resistance to complement activation, cell membrane hypersialylation and relapses in chronic lymphocytic leukemia patients treated with rituximab and chemotherapy

The anti-CD20-specific monoclonal antibody rituximab (RTX), in combination with chemotherapy, is commonly used for primary treatment in chronic lymphocytic leukemia (CLL). However, relapses remain important and activation of the complement pathway is one of the mechanisms by which RTX generates the destruction of B cells directly by complement-dependent cytotoxicity (CDC), or indirectly by antibody-dependent cellular phagocytosis. In this study, the RTX capacity to induce CDC was established in 69 untreated CLL patients, this cohort including 34 patients tested before the initiation of RTX-chemotherapy. In vitro CDC-resistance to RTX predicts lower response rates to RTX-chemotherapy and shorter treatment free survival. Furthermore, the predictive value of CDC-resistance was independent from the clinical, cytogenetic and FcγR3A V158F polymorphism status. In contrast, CLL cell resistance to CDC predominates in IGHV unmutated patients and was related to an important α2-6 sialyl transferase activity, which in turn increases cell surface α2-6 hypersialylation. Suspected factors associated with resistance to CDC (CD20, CD55, CD59, factor H, GM1, and sphingomyelin) were not differentially expressed or recruited between the two CLL groups. Altogether, results provide evidence that testing RTX capacity to induce CDC in vitro represents an independent predictive factor of therapeutic effects of RTX, and that α2-6 hypersialylation in CLL cells controls RTX response through the control of the complement pathway. At a time when CLL therapy is moving towards chemo-free treatments, further experiments are required to determine whether performing an initial in vitro assay to appreciate CLL CDC resistance might be useful to select patients.

Complement system activation contributes to the ependymal damage induced by microbial neuraminidase

Background

In the rat brain, a single intracerebroventricular injection of neuraminidase from Clostridium perfringens induces ependymal detachment and death. This injury occurs before the infiltration of inflammatory blood cells; some reports implicate the complement system as a cause of these injuries. Here, we set out to test the role of complement.

Methods

The assembly of the complement membrane attack complex on the ependymal epithelium of rats injected with neuraminidase was analyzed by immunohistochemistry. Complement activation, triggered by neuraminidase, and the participation of different activation pathways were analyzed by Western blot. In vitro studies used primary cultures of ependymal cells and explants of the septal ventricular wall. In these models, ependymal cells were exposed to neuraminidase in the presence or absence of complement, and their viability was assessed by observing beating of cilia or by trypan blue staining. The role of complement in ependymal damage induced by neuraminidase was analyzed in vivo in two rat models of complement blockade: systemic inhibition of C5 by using a function blocking antibody and testing in C6-deficient rats.

Results

The complement membrane attack complex immunolocalized on the ependymal surface in rats injected intracerebroventricularly with neuraminidase. C3 activation fragments were found in serum and cerebrospinal fluid of rats treated with neuraminidase, suggesting that neuraminidase itself activates complement. In ventricular wall explants and isolated ependymal cells, treatment with neuraminidase alone induced ependymal cell death; however, the addition of complement caused increased cell death and disorganization of the ependymal epithelium. In rats treated with anti-C5 and in C6-deficient rats, intracerebroventricular injection of neuraminidase provoked reduced ependymal alterations compared to non-treated or control rats. Immunohistochemistry confirmed the absence of membrane attack complex on the ependymal surfaces of neuraminidase-exposed rats treated with anti-C5 or deficient in C6.

Conclusions

These results demonstrate that the complement system contributes to ependymal damage and death caused by neuraminidase. However, neuraminidase alone can induce moderate ependymal damage without the aid of complement.

Complement resistance of human carcinoma cells depends on membrane regulatory proteins, protein kinases and sialic acid

Nucleated cells employ several strategies to evade killing by homologous complement. We studied complement resistance in the human carcinoma cell lines (CA) T47D (mammary), SKOV3 (ovarian), and PC-3 (prostate) with emphasis on the following mechanisms of defense: 1. Expression and shedding of the membrane complement regulatory proteins (mCRP) CD46, CD55 and CD59; 2. Resistance based on protein phosphorylation; 3. Cell surface expression of sialic acid residues; 4. Desensitization to complement upon exposure to sublytic complement doses. Anti-mCRP antibody blocking experiments demonstrated that CD59 is the main mCRP protecting these CA from complement. Soluble CD59 was also found in supernates of PC-3> SKOV3 > T47D cells. Second, inhibitors of PKC, PKA and MEK sensitized the CA to lysis, thus implicating these protein kinases in CA complement resistance. Third, removal of sialic acid residues with neuraminidase also sensitized CA to lysis. Finally, exposure of CA to sublytic doses of complement conferred on them enhanced resistance to lytic complement doses in a PKC-dependent process. Combined treatment of CA with anti-CD59 antibodies, PD98059 (a MEK inhibitor) and neuraminidase produced a large enhancement in CA sensitivity to complement. Our results show that CD59 and sialic acid residues present on the cell surface, and intracellular processes involving protein phosphorylation act additively to secure CA resistance to complement-mediated lysis. Therefore, the effectiveness of antibody- and complement-based cancer immunotherapy will markedly improve by suppression of the various complement resistance mechanisms.

Multiple ligand binding sites on domain seven of human complement factor H

Foreign particles and damaged host cells can activate the complement system leading to their destruction by the host defense system. Factor H (fH) plays a vital role in restricting complement activation on host cells through interactions with polyanions such as heparin, while allowing activation to proceed on foreign surfaces. Complement activation by damaged host cells is also down regulated by fH, which is localized to injured areas through interactions with C-reactive protein (CRP). A number of pathogens have developed mechanisms by which they can also bind fH and thus exploit its protective properties. One such organism is Group A Streptococcus (GAS) which mediates fH binding via its surface expressed M-protein. fH consists of 20 conserved short consensus repeat (SCR) units and mutagenesis studies indicate that the seventh repeat is responsible for interactions with heparin, CRP and M-protein. We recently performed molecular modelling of fH SCR 7 and identified a cluster of positively charged residues on one face of the domain. By alanine replacement mutagenesis, we demonstrated that these residues are involved in heparin, CRP and M protein binding, which indicates that there is a common site within fH SCR 7 responsible for multiple ligand recognition.

Single-channel analysis of the conductance fluctuations induced in lipid bilayer membranes by complement proteins C5b-9

Single-channel analysis of electrical fluctuations induced in planar bilayer membranes by the purified human complement proteins C5b6, C7, C8, and C9 have been analyzed. Reconstitution experiments with lipid bilayer membranes showed that the C5b-9 proteins formed pores only if all proteins were present at one side of the membrane. The complement pores had an average single-channel conductance of 3.1 nS at 0.15 M KCl. The histogram of the complement pores suggested a substantial variation of the size of the single channel. The linear relationship between single-channel conductance at fixed ionic strength and the aqueous mobility of the ions in the bulk aqueous phase indicated that the ions move inside the complement pore in a manner similar to the way they move in the aqueous phase. The minimum diameter of the pores as judged from the conductance data is approximately 3 nm. The complement channels showed no apparent voltage control or regulation up to transmembrane potentials of 100 mV. At neutral pH the pore is three to four times more permeable for alkali ions than for chloride, which may be explained by the existence of fixed negatively charged groups in or near the pore. The significance of these observations to current molecular models of the membrane lesion formed by these cytolytic serum proteins is considered.

Cyanine dye fluorescence used to measure membrane potential changes due to the assembly of complement proteins C5b-9

The fluorescent potentiometric indicator diS-C3-(5) has been used to investigate changes in membrane potential due to assembly of the C5b-9 membrane attack complex of the complement system. EAC1-7 human red blood cells and resealed erythrocyte ghosts--bearing membrane-assembled C5b67 complexes--were generated by immune activation in C8-deficient human serum. Studies performed with these cellular intermediates revealed that the membrane potential of EAC1-7 red cells and ghosts is unchanged from control red cells (-7 mV) and ghosts (O mV), respectively. Addition of complement proteins C8 and C9 to EAC1-7 red cells results in a dose-dependent depolarization of membrane potential which precedes hemolysis. This prelytic depolarization of membrane potential--and the consequent onset of hemolysis--is accelerated by raising external [K+], suggesting that the diffusional equilibration of transmembrane cation gradients is rate limiting to the cytolytic event. In the case of EAC1-7 resealed ghosts suspended at either high external [K+] or [Na+], no change in membrane potential (from O mV) could be detected after C8/C9 additions. When the membrane potential of the EAC1-7 ghost was displaced from O mV by selectively increasing the K+ conductance with valinomycin, a dose-dependent depolarization of the membrane was observed upon addition of C8 and C9. In these experiments, lytic breakdown of the ghost membranes was less than 5%. Conclusions derived from this study include: (i) measured prelytic depolarization of the red cell Donnan potential directly confirms the colloid-osmotic theory of immune cytolysis. (ii) The diffusional transmembrane equilibration of Na+ and K+ through the C5b-9 pore results in a dose-dependent depolarization of the membrane potential (Em) which appears to be rate-limiting to cytolytic rupture of the target erythrocyte. (iii) Enhanced immune hemolysis observed in high K+ media cannot be attributed to cation-selective conductance across the C5b-9 pore, and is probably related to the near-equilibrium condition of potassium-containing red cells when suspended at high external K+. These experiments demonstrate that carbocyanine dye fluorescent indicators can be used to monitor electrochemical changes arising from immune damage to the plasma membrane under both cytolytic and noncytolytic conditions. Potential application of this method to the detection of sublytic pathophysiological changes in the plasma membrane of complement-damaged cells are discussed.

Proteolytic modification of human complement protein C9: loss of poly(C9) and circular lesion formation without impairment of function

We have compared the ability of thrombin-cleaved C9 (C9n) with that of native C9 to produce tubular or ring-like poly(C9) and to express the classical complement lesion on target membranes. Three procedures were used to produce poly(C9): (i) limited proteolysis with trypsin, (ii) interaction with small unilamellar lipid vesicles, and (iii) incubation with a 2- to 4-fold molar excess of ZnCl2. In contrast to C9, which could be converted to tubular poly(C9), C9n was converted to smaller peptides by the first procedure and was aggregated into string-like poly(C9) by the other two methods. C9-depleted human serum (R-9 serum) was reconstituted with either C9 or C9n and these sera were then used to lyse sensitized sheep erythrocytes. Numerous classical complement lesions could be detected on ghost membranes obtained from cells lysed by C9-reconstituted R-9 serum but only a few on ghost membranes produced by C9n-reconstituted R-9 serum. C9n was shown to be hemolytically as active as C9 even when tested under "single-hit" conditions and it was about twice as efficient when compared with C9 in releasing sucrose and inulin from resealed ghosts. These results are interpreted to indicate that formation of the classical complement lesion is only incidental to lysis and not an obligatory event and that enlargement of the "functional pore size" of the complement lesion is not linked to formation of a circular membrane attack complex.

The influence of electrochemical gradients of Na+ and K+ upon the membrane binding and pore forming activity of the terminal complement proteins

The hemolytic activity of the terminal complement proteins (C5b-9) towards erythrocytes containing high potassium concentration has been reported to be dramatically increased when extracellular Na+ is substituted isotonically by K+ (Dalmasso, A.P., et al., 1975, J. Immunol. 115:63-68). This phenomenon was now further investigated using resealed human erythrocyte ghosts (ghosts), which can be maintained at a nonlytic osmotic steady state subsequent to C5b-9 binding: (1) The functional state of C5b-9-treated ghosts was studied from their ability to retain trapped [14C]-sucrose or [3H]-inulin when suspended either in the presence of Na+ or K+. A dramatic increase in the permeability of the ghost membrane to both nonelectrolytes - in the absence of significant hemoglobin release - was observed for C5b-9 assembly in the presence of external K+. (2) The physical binding of the individual 125I-labeled terminal complement proteins to ghost membranes was directly measured as a function of intra- and extracellular K+ and Na+. The uptake of 125I-C7, 125I-C8, and 125I-C9 into membrane C5b-9 was unaltered by substitution of Na+ by K+. (3) The binding of the terminal complement proteins to ghosts subjected to a transient membrane potential generated by the K+-ionophore valinomycin (in the presence of K+ concentration gradients) was measured. No significant change in membrane binding of any of the C5b-9 proteins was detected under the influence of both depolarizing and hyperpolarizing membrane potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Complement pores in erythrocyte membranes. Analysis of C8/C9 binding required for functional membrane damage

The number of membrane-bound terminal complement proteins (C5b-9) required to generate a functional pore in the human erythrocyte membrane ghost has been determined. Resealed erythrocyte ghost membranes (ghosts) were treated with human complement proteins C5b6, C7, 131I-C8, and 125I-C9 under non-lytic conditions. Following C5b-9 assembly, sucrose-permeant ghosts were separated from C5b-9 ghosts that remained impermeant to sucrose by centrifugation over density barriers formed of 43% (w/v) sucrose. Analysis of 131I-C8 and 125I-C9 bound to sucrose-permeant and sucrose-impermeant subpopulations of C5b-9 ghosts revealed: 1. Sucrose-permeant C5b-9 ghosts show increased uptake of both 131I-C8 and 125I-C9 as compared to ghosts that remain impermeant to sucrose. Ghosts with less than 300 molecules 131I-C8 bound remain impermeant to sucrose, irrespective of the total C9 input, or, the multiplicity of C9 uptake by membrane C5b-8. 2. In the presence of excess 125I-C9, the ratio of 125I-C9/131I-C8 bound to membrane C5b67 is 3.2 +/- 0.8 (mean +/- 2 S.D.), suggesting an average stoichiometry of 3 C9 per C5b-8. Under these conditions, the ratio of 125I-C9/131I-C8 bound to sucrose-permeant ghosts (3.3 +/- 0.7) does not significantly differ from the ratio bound to sucrose-impermeant ghosts (2.9 +/- 0.6). 3. With limiting C9 input, the threshold of total C5b-8 uptake required for sucrose permeability increases significantly above 300 per cell when the ratio of bound 125I-C9/131I-C8 is decreased below unity. In the complete absence of C9, 11 700 C5b-8 complexes are bound to sucrose-permeant ghosts. It is concluded that more than 300 C5b-9 complexes must bind to the human erythrocyte to form a sucrose-permeant lesion. Although the binding of one C9 per C5b-8 is critical to the pore-forming activity of these proteins, the binding of additional molecules of C9 to each complex (C9/C8 greater than 1) does not significantly alter the threshold of total C5b-9 uptake required for lesion formation.

Bactericidal and bacteriolytic activity of serum against gram-negative bacteria

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Effect of agents that produce membrane disorder on lysis of erythrocytes by complement

To evaluate the effect of membrane lipid acyl-chain packing on the efficiency of cell lysis by complement, we have studied membrane modulation by 2-(2-methoxy)-ethoxyethyl-8-(cis-2-n-octylcyclopropyl)-octanoate (A2C) and by myristoleyl alcohol, the cis isomer of a C14:1 aliphatic alcohol. These substances are known to increase the membrane lipid disorder by virtue of the bend in their acyl chains, which is believed to loosen the phospholipid acyl-chain packing. We have found that both of these compounds markedly enhance the lysis of erythrocytes by the terminal complement proteins C5b-9. The enhancing effect by A2C is operative in the formation of erythrocytes carrying complement components C5b, C6, and C7, as well as in the subsequent reactions with complement components C8 and C9. We have also found that A2C-treated erythrocytes bind C5b6 to a measurable extent, whereas untreated erythrocytes do not. We attribute this to a shift in the partition equilibrium of C5b6 toward membrane association, which would improve lytic efficiency. The increase of membrane lipid disorder by these agents would also be expected to increase insertion of hydrophobic peptides from C7, C8, and C9, with consequent gain in lytic efficiency. Treatment of erythrocytes with sublytic doses of NaDodSO4, or Triton X-100 did not enhance lysis by C5b-9 appreciably, suggesting that enhancement of lysis by C5b-9 is not a general property of amphiphiles.

Permeability characteristics of complement-damaged membranes: evaluation of the membrane leak generated by the complement proteins C5b-9

Permeability characteristics of the membrane lesion generated by the terminal complement proteins are considered in light of recent observations that the measured diffusion of solute across complement-damaged membranes does not conform to the "doughnut hole" model of a discrete transmembrane pore formed by the inserted C5b-9 complex. By using the measured kinetics of steady-state tracer isotope diffusion of nonelectrolytes across resealed erythrocyte ghost membranes treated with C5b-9, a new transport model is developed. This model considers the apparent membrane lesion strictly in terms of the operational criteria of a functional conducting pathway for the observed diffusing solute, independent of a priori assumptions about the geometry or molecular properties of the membrane lesion. With this definition of the unit membrane lesion and the assumption that the exclusion size of the conducting pathway varies directly with the multiplicity of bound C5b-9 (as suggested by previous measurements under conditions of varying input of C5b-9), numerical estimates of te apparent permeability of the complement-damaged membrane to four diffusing nonelectrolytes are derived. These results suggest that the pathway for a particle diffusing across the complement lesion cannot be a pore and is functionally equivalent to an aqueous leak pathway, free of pore constraints. Implications of these results are discussed in terms of current molecular models for the mechanism of membrane damage by the complement proteins.

Steady-state analysis of tracer exchange across the C5b-9 complement lesion in a biological membrane

Resealed erythrocyte ghosts have been used to define the kinetics of tracer exchange across the membrane-bound terminal complex of the complement cascade (C5b-9). Under steady-state conditions and at net chemical equilibrium, C5b-9 ghosts showed no significant lysis above control levels as measured by hemoglobin efflux. In 1 mM sucrose at 37 degrees C, [14C]sucrose isotopic exchange diffusion into C5b-9 ghosts occurred at 4.8 (+/- 0.5, SEM) X 10(-20) mol sec-1 per functional lesion, equivalent to an apparent permeability coefficient of 4.8 X 10(-14) cm3 sec-1 for the single C5b-9 lesion. No significant uptake of [14C]sucrose above control levels was observed in C5b67 ghosts. The apparent rate of tracer permeation through the complement lesion is one to two orders of magnitude slower than predicted by a model of a transmembrane channel of dimensions permitting free diffusion of sucrose. The data support earlier assertions from this laboratory that diffusion of small molecules across the complement lesion in biological membranes is significantly restricted.

Regulation by membrane sialic acid of beta1H-dependent decay-dissociation of amplification C3 convertase of the alternative complement pathway

Sheep erythrocytes in their native state did not activate the alternative complement pathway, as measured by lysis in dilutions of normal human serum containing [ethylenebis(oxyethylenenitrilo)] tetraacetic acid but acquired this capacity after membrane sialic acid residues had been removed (by sialidase) or modified (by NaIO(4)). Activation of the alternative pathway by sheep erythrocytes required removal or modification of at least 40% of the membrane sialic acid to reach threshold, and it increased proportionately when larger amounts of sialic acid had been affected. Studies with isolated proteins of the alternative pathway demonstrated that the altered erythrocyte membranes resembled natural activators in protecting bound C3b from inactivation by C3b inactivator and beta1H and protecting bound amplification C3 convertase (C3b,Bb) from decay-dissociation by beta1H. A 1% decrease in intact sialic acid was associated with a 1% decrease in beta1H activity in decay-dissociation of membrane bound C3b,Bb. Because removal of the C8 and C9 carbon atoms from the polyhydroxylated side chain of sialic acid by oxidation with NaIO(4) was functionally equivalent to removal of the entire sialic acid moiety, secondary effects of the latter reaction, such as diminution of the negative charge of the membrane or exposure of penultimate galactose residues, were not considered to be responsible for the altered activity of beta1H. These studies suggest that facilitation, by membrane sialic acid residues, of the interaction between bound C3b and beta1H is essential to prevent the particle from effectively activating the alternative pathway.

Complement lysis: the ultrastructure and orientation of the C5b-9 complex on target sheep erythrocyte membranes

The C5b-9 complex derived from human serum and assembled on target sheep erythrocyte membranes is a thin-walled cylinder rimmed by an annulus at one end. The total height of the cylinder is 150 A, towards which the annulus contributes 30 A. The cylinder has an apparently uniform internal diameter of 100 A. The external diameter of the annulus is 200 A. The classical complement 'rings' visualized on membranes after complement lysis represent such C5b-9 cylinders perpendicularly oriented on the membranes. The thin-walled cylinder is anchored in the membrane matrix and the annulus located in the exterior membrane glycocalyx. At the sites of attachment of the C5b-9 complexes, the continuity of the membrane bilayer is disturbed and the presence of trans-membrane pores is indicated. The data essentially support the 'doughnut' theory of complement lysis.

Binding characteristics of M and L isoantibodies to high and low potassium sheep red cells

Binding of highly purified 125I labeled M and L antibodies, both belonging to the immunoglobulin G class, was studied in high potassium (HK) and low potassium (LK) sheep red cells. Anti-M and anti-L bound specifically to M and L antigen positive HK and LK red cells, respectively. Nonspecific binding was higher for anti-L to HK cells than for anti-M to LK cells. Once bound, the M and L antibodies were capable of inducing complement dependent immune hemolysis. Only 75-100 and 500-750 molecules of anti-M and anti-L immunoglobulins were required to hemolyze 50% of HK (MM) and LK (LL) red cells, respectively, suggesting that the M and L antigens may be clustered on the surfaces of these cells. Equilibrium binding studies revealed that the maximum number of M sites is 3-6 x 10(3) in HK (MM) and 1.5-4 x 10(3) in LK (LM) cells, respectively. In comparison, the number of L antigens is slightly lower in LK cells, about 1.2-1.8 x 10(3) in LL and less in LM(LK) red cells. The number of M and L antigens, therefore, is more than an order of magnitude larger than that of the Na+K+ pumps measured previously in these cells by 3H-ouabain binding, thus precluding a quantitative correlation between M and L antigens and the Na+K+ pumps different in the three genetic types of sheep red cells. The binding affinities of both anti-M and anti-L could not be described by a single equilibrium dissociation constant indicating heterogeneous antibody populations and /or variability in the antigenic sets of individual HK or LK cells. The pronounced heterogeneity of antigens and/or antibodies in both the M and L systems was reflected in the antibody association kinetics, which also exhibited a remarkable temperature dependence. The data suggest that the correlation between the M and L antigens and the Na+K+ pump molecules is more complex than that in goat red cells previously reported by others.