Increased production of tumor necrosis factor-alpha in whole blood cultures from children with primary malnutrition.

Because low tumor necrosis factor-alpha (TNF-alpha) production has been reported in malnourished children, in contrast with high production of TNF-alpha in experimental protein-energy malnutrition, we reevaluated the production of TNF-alpha in whole blood cultures from children with primary malnutrition free from infection, and in healthy sex- and age-matched controls. Mononuclear cells in blood diluted 1:5 in endotoxin-free medium released TNF-alpha for 24 h. Spontaneously released TNF-alpha levels (mean +/- SEM), as measured by enzyme immunoassay in the supernatants of unstimulated 24-h cultures, were 10,941 +/- 2,591 pg/ml in children with malnutrition (N = 11) and 533 +/- 267 pg/ml in controls (N = 18) (P < 0.0001). TNF-alpha production was increased by stimulation with lipopolysaccharide (LPS), with maximal production of 67,341 +/- 16,580 pg/ml TNF-alpha in malnourished children and 25,198 +/- 2,493 pg/ml in controls (P = 0.002). In control subjects, LPS dose-dependently induced TNF-alpha production, with maximal responses obtained at 2000 ng/ml. In contrast, malnourished patients produced significantly more TNF-alpha with 0.02-200 ng/ml LPS, responded maximally at a 10-fold lower LPS concentration (200 ng/ml), and presented high-dose inhibition at 2000 ng/ml. TNF-alpha production a) was significantly influenced by LPS concentration in control subjects, but not in malnourished children, who responded strongly to very low LPS concentrations, and b) presented a significant, negative correlation (r = -0.703, P = 0.023) between spontaneous release and the LPS concentration that elicited maximal responses in malnourished patients. These findings indicate that malnourished children are not deficient in TNF-alpha production, and suggest that their cells are primed for increased TNF-alpha production.

Increased production of tumor necrosis factor-alpha in whole blood cultures from children with primary malnutrition

Because low tumor necrosis factor-alpha (TNF-alpha) production has been reported in malnourished children, in contrast with high production of TNF-alpha in experimental protein-energy malnutrition, we reevaluated the production of TNF-alpha in whole blood cultures from children with primary malnutrition free from infection, and in healthy sex- and age-matched controls. Mononuclear cells in blood diluted 1:5 in endotoxin-free medium released TNF-alpha for 24 h. Spontaneously released TNF-alpha levels (mean ± SEM), as measured by enzyme immunoassay in the supernatants of unstimulated 24-h cultures, were 10,941 ± 2,591 pg/ml in children with malnutrition (N = 11) and 533 ± 267 pg/ml in controls (N = 18) (P < 0.0001). TNF-alpha production was increased by stimulation with lipopolysaccharide (LPS), with maximal production of 67,341 ± 16,580 pg/ml TNF-alpha in malnourished children and 25,198 ± 2,493 pg/ml in controls (P = 0.002). In control subjects, LPS dose-dependently induced TNF-alpha production, with maximal responses obtained at 2000 ng/ml. In contrast, malnourished patients produced significantly more TNF-alpha with 0.02-200 ng/ml LPS, responded maximally at a 10-fold lower LPS concentration (200 ng/ml), and presented high-dose inhibition at 2000 ng/ml. TNF-alpha production a) was significantly influenced by LPS concentration in control subjects, but not in malnourished children, who responded strongly to very low LPS concentrations, and b) presented a significant, negative correlation (r = -0.703, P = 0.023) between spontaneous release and the LPS concentration that elicited maximal responses in malnourished patients. These findings indicate that malnourished children are not deficient in TNF-alpha production, and suggest that their cells are primed for increased TNF-alpha production.

[In vitro effects of antiallergic eyedrops on complement activation induced by particulate matter]. / Effets modulateurs de collyres anti-allergiques sur l'activation du complément induite in vitro par des polluants particulaires.

BACKGROUND: Recent decades have been marked by an increasing number of patients suffering from ocular allergic-like symptoms without being associated with an increase in IgE levels. These symptoms include heaviness of the lid, foreign body sensation, burning, stinging and photophobia. Both epidemiological studies and controlled human exposure clinical studies have shown cause-effect relationships between allergic-like symptoms and environmental factors such as outdoor air pollutants or poor indoor air quality. An ocular surface subclinical inflammation is thought to be responsible for pseudoallergic, pollution-related conjunctivitis. The complement system is considered as one of the major effector mechanisms involved in initiation of the subclinical inflammation that leads to IgE-independent eye irritation. PURPOSE: To study the capability of nine antiallergic eyedrops commonly used in the treatment of allergic conjunctivitis to inhibit complement activation induced in vitro by pollutants. METHODS: Normal human serum obtained from healthy individuals was used as a source of complement. Activation of complement was assessed using the complement hemolytic 50% (CH50) assay, in the absence or the presence of antiallergic eyedrops and in the absence or the presence of various stimuli, including sand, common house dust, eye mascara, and Dactylis glomerata pollen extract. Zymosan was used as a standardized complement activator. The following eyedrops were studied: Naabak (4.9% N-acetyl aspartic acid-glutamic acid, NAAGA, sodium salt), Almide (lodoxamide 0.1%), Levophta (0.05% levocabastine), Emadine (0.05% emedastine), Tilavist (2% nedocromil), Allergodil (0.05% azelastine), Patanol (olopatadine), and Zaditen (0.025% ketotifen). Effects of preservative-free lodoxamide and ketotifen were also assessed and compared to those of the preserved formulations. A solution of 0.01% benzalkonium chloride (BAC), the most widely used preservative in topical eyedrops, was also tested. RESULTS: Zymosan-induced activation of complement (30+/-6%) was significantly lowered by preincubation of serum with unpreserved NAAGA (16.6+/-4%, p=0.0026) or benzalkonium-preserved nedocromil (20+/-2%, p=0.022). Preserved levocabastine, emedastine, olopatadine and ketotifen did not interfere with zymosan-induced complement activation, whereas preserved azelastine, lodoxamide and benzalkonium chloride significantly aggravated complement activation induced by zymosan. Similar results were obtained when complement activation was triggered by sand, common house dust, mascara, or by an allergenic extract of Dactylis glomerata pollen. In the absence of complement activator, none of the antiallergic eyedrops induced a significant change in CH50 titer, indicating that the deleterious pro-inflammatory effect of preserved azelastine and lodoxamide may occur only once complement activation has been initiated, i.e., on an inflamed ocular surface. CONCLUSION: Among the antiallergic eyedrops tested in this study, only Naabak and Tilavist were found to significantly inhibit complement activation triggered by particulate matters or pollen allergenic extract. Such an anticomplement activity confers these two molecules a potential in the therapeutic management of pollution-related pseudoallergic conjunctivitis.

[N acetyl-aspartyl glutamic acid (NAAGA) inhibits the adhesion of leukocytes to activated endothelial cells and down-modulates the cytokine-induced expression of adhesion molecules]. / Effets inhibiteurs du dipeptide N-acétyl-aspartyl-glutamate (NAAGA) sur l'adhérence des leucocytes humains aux cellules endothéliales activées et l'expression des molécules d'adhésion CD11b, CD49d, ICAM-1, ICAM-2, VCAM-1 et ELAM-1.

BACKGROUND: Cell adhesion plays a pivotal role in most ocular surface inflammatory diseases. Adhesion molecules mediate cell-to-cell and cell-to-matrix adhesion. Their expression is up-regulated by pro-inflammatory stimuli such as cytokines, histamine or complement-derived anaphylatoxins. The dipeptide N acetyl-aspartyl glutamic acid (NAAGA) is used as unpreserved topical eyedrops in the treatment of allergic conjunctivitis. NAAGA is known to inhibit leukotriene synthesis, histamine release by mast cells, and complement-derived anaphylatoxin production. PURPOSE: To investigate the potential capability of NAAGA to interfere with leukocyte adhesion to endothelial cells and modulate cytokine-induced expression of adhesion molecules. METHODS: Human blood-derived leukocytes were co-cultured with human umbilical vein endothelial cells (HUVECs) in the absence or the presence of 1000 UI/mL human recombinant TNFalpha, 10(-4) M histamine di-hydrochloride or 5x10(-6) M human recombinant C5a, and in the absence or presence of NAAGA (final concentration 2.45%). Adhesion of leukocytes to HUVECs was calculated by subtracting the number of nonadherent leukocytes from the total number of leukocytes. Expression of adhesion molecules was assessed by flow cytometry using anti-CD11b, anti-CD49d, anti-ICAM-1 (CD54), anti-ICAM-2 (CD102), anti-VCAM-1 (CD106) and anti-ELAM-1 (CD62E) monoclonal antibodies. RESULTS: NAAGA was found to totally inhibit adhesion of unstimulated leukocytes, or leukocytes activated with C5a, TNFalpha, or histamine, to TNFalpha-stimulated HUVECs (P=0.0001). Adhesion of leukocytes to unstimulated HUVECs was not modified by NAAGA. Similar results were obtained with endothelial cells stimulated by histamine or C5a. Taken together, these data indicate that NAAGA totally abrogates cell adhesion under inflammatory conditions, without interfering with the physiological adhesion of leukocytes to normal endothelium. At the molecular level, NAAGA inhibited histamine-induced expression of CD11b (P=0.0004) and CD49d (P=0.0045) on granulocytes. On TNFalpha-activated HUVECs, NAAGA induced a significant decrease in the VCAM-1 expression level (P<0.0001) and totally reversed TNFalpha-induced overexpression of ICAM-1 (P=0.0069), ICAM-2 and ELAM-1 (P<0.0001), without interfering with baseline expression of these molecules. CONCLUSION: These results show that the antiallergic compound NAAGA directly inhibits leukocyte adhesion to endothelial cells induced by pro-inflammatory stimuli, and abrogates TNFalpha-induced expression of adhesion molecules on granulocytes and endothelial cells. This capacity to block overexpression of selectins and integrins induced by pro-inflammatory stimuli confers to NAAGA a potential as an anti-inflammatory drug, since interfering with adhesion molecule expression is probably one of the most efficient ways to curb leukocyte recruitment to inflammatory sites.

Gram-positive and gram-negative bacteria do not trigger monocytic cytokine production through similar intracellular pathways.

Toll-like receptors (TLRs) are involved in human monocyte activation by lipopolysaccharide (LPS) and Staphylococcus aureus Cowan (SAC), suggesting that gram-positive and gram-negative bacteria may trigger similar intracellular events. Treatment with specific kinase inhibitors prior to cell stimulation dramatically decreased LPS-induced cytokine production. Blocking of the p38 pathway prior to LPS stimulation decreased interleukin-1alpha (IL-1alpha), IL-1ra, and tumor necrosis factor alpha (TNF-alpha) production, whereas blocking of the ERK1/2 pathways inhibited IL-1alpha, IL-1beta, and IL-1ra but not TNF-alpha production. When cells were stimulated by SAC, inhibition of the p38 pathway did not affect cytokine production, whereas only IL-1alpha production was decreased in the presence of ERK kinase inhibitor. We also demonstrated that although LPS and SAC have been shown to bind to CD14 before transmitting signals to TLR4 and TLR2, respectively, internalization of CD14 occurred only in monocytes triggered by LPS. Pretreatment of the cells with SB203580, U0126, or a mixture of both inhibitors did not affect internalization of CD14. Altogether, these results suggest that TLR2 signaling does not involve p38 mitogen-activated protein kinase signaling pathways, indicating that divergent pathways are triggered by gram-positive and gram-negative bacteria, thereby inducing cytokine production.

Antibodies to C-C chemokine receptor 5 in normal human IgG block infection of macrophages and lymphocytes with primary R5-tropic strains of HIV-1.

In the present study, we demonstrate that normal human IgG for therapeutic use (i.v. Ig) contains natural Abs directed against the CCR5 coreceptor for HIV-1. Abs to CCR5 were isolated from i.v. Ig using an affinity matrix consisting of a synthetic peptide corresponding to the N-terminus of CCR5 coupled to Sepharose. Natural anti-CCR5 Abs inhibited the binding of RANTES to macrophages, demonstrating their interaction with the coreceptor of R5-tropic HIV-1. Affinity-purified anti-CCR5 Ig further inhibited infection of lymphocytes and monocytes/macrophages with primary and laboratory-adapted strains of HIV-1, but did not inhibit infection with X4-tropic HIV. Our results suggest that anti-CCR5 Abs from healthy immunocompetent donors may be suitable for development of novel passive immunotherapy regimens in specific clinical settings in HIV infection.

Opposite effects of IL-10 on the ability of dendritic cells and macrophages to replicate primary CXCR4-dependent HIV-1 strains.

We investigated the effect of IL-10 on replication of primary CXCR4-dependent (X4) HIV-1 strains by monocyte-derived dendritic cells (DCs) and macrophages (M Phis). M Phis efficiently replicated CXCR4-dependent HIV-1 (X4 HIV-1) strains NDK and VN44, whereas low levels of p24 were detected in supernatants of infected DCs. IL-10 significantly increased X4 HIV-1 replication by DCs but blocked viral production by M Phis as determined by p24 levels and semiquantitative nested PCR. IL-10 up-regulated CXCR4 mRNA and protein expression on DCs and M Phis, suggesting that IL-10 enhances virus entry in DCs but blocks an entry and/or postentry step in M Phis. The effect of IL-10 on the ability of DCs and M Phis to transmit virus to autologous CD4(+) T lymphocytes was investigated in coculture experiments. DCs exhibited a greater ability than did M Phis to transmit a vigorous infection to CD4(+) T cells despite their very low replication capacity. IL-10 had no effect on HIV-1 replication in DC:T cell cocultures but markedly decreased viral production in M Phi:T cell cocultures. These results demonstrate that IL-10 has opposite effects on the replication of primary X4 HIV-1 strains by DCs and M Phis. IL-10 increases X4-HIV-1 replication in DCs but does not alter their capacity to transmit virus to CD4(+) T lymphocytes. These findings suggest that increased levels of IL-10 observed in HIV-1-infected patients with disease progression may favor the replication of X4 HIV-1 strains in vivo.

Soluble CD16 inhibits CR3 (CD11b/CD18)-mediated infection of monocytes/macrophages by opsonized primary R5 HIV-1.

We demonstrate that soluble CD16 (sCD16; soluble Fc gamma RIII), a natural ligand of CR3, inhibits the infection of monocytes by primary R5 HIV-1 strain opsonized with serum of seronegative individuals. Inhibition of monocyte infection by sCD16 was similar to that observed with anti-CR3 mAbs, indicating that opsonized HIV may use a CR3-dependent pathway for entry in monocytic cells. Cultured human monocytes express both CR3 (CD11b/CD18) and CCR5 receptors. RANTES, the natural ligand of CCR5, inhibited infection of monocytes with unopsonized HIV particles and partially that of monocytes infected with HIV particles opsonized with complement-derived fragments. Although HIV-infected monocytes from homozygous CCR5 Delta 32/Delta 32 (CCR5(-/-)) individuals produce low levels of p24, cells infected with opsonized particles produced higher levels of p24 than cells infected with unopsonized particles. Our results thus suggest that CR3 may represent an alternative coreceptor to CCR5 of opsonized primary R5 virus entry into monocytes/macrophages. We also observed that the concentration of sCD16 is greatly decreased in sera of HIV-infected patients with low lymphocyte CD4(+) counts. Taken together, our findings suggest that sCD16, present in plasma, may play an important role in controlling HIV-1 spread.

CD14+CD16++ cells derived in vitro from peripheral blood monocytes exhibit phenotypic and functional dendritic cell-like characteristics.

We previously reported an increased percentage of CD14+CD16++ monocytes in the peripheral blood of HIV-infected patients but the physiopathological role of this monocyte subset remains unclear. Cells with a CD14+CD16++ phenotype may be obtained in vitro by culturing human peripheral blood monocytes in the presence of GM-CSF, IL-4 and IL-10. In the present study, we compared the phenotypic and functional characteristics of monocytes-derived CD14+CD16++ cells with those of macrophages and dendritic cells. We show that the CD14+CD16++ cells express dendritic cell markers: CD40, CD80, CD86, HLA-DR, CD11b, CD11c, CD18, CD1a, and CD83. Using RNase protection assay, we demonstrate that CD14+CD16++ cell subset expresses a low ratio of IL-1beta/IL-1ra mRNA and expresses IL-6, MIP-1alpha, MIP-1beta, MCP-1, IL-8, RANTES and I-309 transcripts, similar to dendritic cells. CD14+CD16++ cells produce IL-12, MCP-1 and IL-8, as assessed by flow cytometry. Moreover, CD14+CD16++ cells pulsed with different recall antigens induce a potent autologous T cell proliferation. Altogether, these results provide evidence that CD14+CD16++ cells differentiated in vitro from peripheral blood monocytes exhibit dendritic cell characteristics.

In vivo intracellular cytokine production by leukocytes during haemodialysis.

Several chronic inflammatory changes undergone during chronic haemodialysis are associated with increased pro-inflammatory cytokine production. Although generation of anaphylatoxins has been incriminated in the untoward effects of haemodialysis, it is still debated whether anaphylatoxins stimulate monocyte secretion of TNF-alpha and IL-1. We demonstrate that peripheral mononuclear cells isolated from healthy controls and cultured with complement-activated autologous serum or recombinant C5a induced high levels of IL-1, IL-1ra, IL-8 and MCP-1, low levels of TNFalpha and sTNFRII but no IL-10 and MIP-1alpha. Cytokine production by leukocytes was investigated by FACS analysis in six patients dialysed consecutively with three equivalent low permeability membranes known to activate the complement to different degrees: polysulfone (F6HPS), cellulose acetate (CA) and cuprophane (CP). Percentage of leukocytes expressing IL-1, IL-1ra, TNF-alpha and IL-8 is increased in patients dialysed with CP. Moreover, we show for the first time that haemodialysis is associated with the production of cytokines by circulating neutrophils. Predialysis plasma levels of MCP-1 and TNFRII did not increase during the dialysis session at the time when anaphylatoxin generation was highest. Dialysis with membranes that activate the complement to a high extent induce activation of leukocytes which may explain chronic complications associated with dialysing with CP.

Dissociation between complement activation, integrin expression and neutropenia during hemodialysis.

Complement activation, neutrophil stimulation, increased cellular adhesiveness, transient leukocyte margination and pulmonary leukostasis take place during hemodialysis with cellulosic dialysis membranes. Several investigators have hypothesized that complement activation is primarily responsible for the acute neutropenia occurring during the early phase of bio-incompatible hemodialysis. We have investigated the relationship between complement activation, levels of expression of CD11b and CD61 integrins on neutrophils and platelets, neutrophil counts and blood gas measurements in patients dialyzed with three types of membranes, known to activate the complement system to a different extent. Polysulfone, cellulose acetate and cuprophane membranes were used subsequently in six patients in a prospective cross-over trial design to reduce inter-individual variability. Increased levels of CD61 and CD11b, as well as neutropenia, were detected regardless of the type of membrane used. We observed a high inter-individual variation with regard to complement activation suggesting varying susceptibility to dialysis membranes. We also report that the kinetics of anaphylatoxin generation were dissociated from those of the upregulation of adhesion molecules, early neutrophil margination and decrease in PaO2 during the first 30 min of hemodialysis. Similar results were obtained with all three types of dialysis membranes. The data strengthen the hypothesis that factors other than complement are involved in the induction of dialysis-related neutropenia and hypoxemia.

Growth hormone prevents human monocytic cells from Fas-mediated apoptosis by up-regulating Bcl-2 expression.

Apoptosis and particularly Fas-mediated apoptosis has been proposed to play a key role in controlling monocyte homeostasis. We and others have documented the regulatory function of human growth hormone (hGH) on monocytic cells, which prompted us to investigate the role of hGH on their response to Fas antigen cross-linking. Using human promonocytic U937 cells constitutively producing hGH upon gene transfer and human primary monocytes cultured in the presence of recombinant hGH, we demonstrated that hGH diminished Fas-mediated cell death by enhancing the expression of the antiapoptotic oncoprotein Bcl-2 as well as the level of bcl-2alpha mRNA. In parallel, we established that overexpression of Bcl-2 through gene transfer into normal U937 cells also diminished Fas-induced apoptosis. Further, as a result of Bcl-2 overexpression, we found that hGH greatly depressed Fas-induced activation of the cysteine protease caspase-3 (CPP32), which in turn affected the cleavage of poly(ADP-ribose) polymerase. Altogether, these data provide evidence that hGH mediates its protective effect through a Bcl-2-dependent pathway, clearly a crucial step in enhanced survival of monocytic cells exposed to Fas-induced death.

Persistent expansion, in a human immunodeficiency virus-infected person, of V beta-restricted CD4+CD8+ T lymphocytes that express cytotoxicity-associated molecules and are committed to produce interferon-gamma and tumor necrosis factor-alpha.

The present study describes the persistent expansion of a subpopulation of circulating double-positive CD4+CD8+ T cells in a human immunodeficiency virus (HIV)-infected person over 8 years. The percentage of double-positive cells was remarkably stable with time and was not related to HIV plasma virus load. CD4+CD8+ cells exhibited phenotypic characteristics of activated memory T lymphocytes. Analysis of V beta usage by the T cell receptors of these cells indicated restricted expression to the V beta 14 and V beta 17 families. Most CD4+CD8+ cells constitutively expressed cytotoxicity-associated molecules (C1.7 and perforin) and were selectively committed to produce interferon-gamma and tumor necrosis factor-alpha, cytokines involved in cytotoxic function. The kinetics of changes in the relative proportion of single-positive CD4+ and double-positive CD4+CD8+ T cell subsets and a similar bias in V beta usage by these subsets suggest that CD4+CD8+ lymphocytes originate from peripheral expansion of mature CD4+ T cells.

Plasma levels of monocyte chemoattractant protein-1 but not those of macrophage inhibitory protein-1alpha and RANTES correlate with virus load in human immunodeficiency virus infection.

Plasma levels of proinflammatory cytokines, cytokine inhibitors, and the beta chemokines RANTES, macrophage inhibitory protein (MIP)-1alpha, and monocyte chemoattractant protein (MCP)-1 were studied in relationship with virus load in 40 patients exhibiting plasma levels of HIV RNA ranging between undetectable and levels >10(6) copies/mL. Mean plasma levels of MCP-1 were increased in patients with high virus load compared with HIV-seropositive subjects with undetectable plasma viral RNA and healthy controls. MCP-1 levels were directly correlated with plasma levels of HIV RNA. No correlation was observed between virus load and plasma concentrations of MIP-1alpha and RANTES. The results suggest that low rates of viral replication in vivo are not dependent on increased production of the suppressive chemokines RANTES and MIP-1alpha. Since MCP-1 upregulates viral replication in vitro, the results may suggest a role for MCP-1 in triggering viral replication in HIV disease.

Dissociation between beta-2 microglobulin and IL-1 production in hemodialyzed patients.

BACKGROUND: beta-2 microglobulin is predominant in amyloid deposits in patients undergoing long term hemodialysis. Amyloid accumulation has been ascribed to dialysis membranes, endotoxin contamination of the dialysate, uremia and chronic systemic inflammation associated with enhanced monocytic cytokine production in hemodialyzed patients. Interleukin-1 has been proposed to play a critical role in the induction of beta-2 microglobulin synthesis and release. METHODS: We examined if monocytes contribute to beta-2 microglobulin production upon stimulation with inflammatory mediators that are generated during hemodialysis and investigated the production of beta-2 microglobulin by cells from patients, with and without clinical signs of amyloidosis, at the time when patients' monocytes contained maximal intracellular accumulation of IL-1. RESULTS: We demonstrated that only monocytes are able to release increased levels of beta-2 microglobulin upon stimulation by IL-1, TNF alpha, C5a and LPS. Increased levels of beta-2 microglobulin were associated with increased levels of beta-2 microglobulin mRNA. Before dialysis session, 20-60% of circulating CD14+ monocytes from patients contained IL-1. At the time when maximal IL-1 production was detected, we showed by RT-PCR increased transcription of IL-1 gene in patients' monocytes. We observed that monocytes from patients with amyloidosis contained higher amounts of IL-1 as compared to monocytes from patients without clinical signs of amyloidosis, but could not secrete increased amounts of beta-2 microglobulin upon LPS-stimulation. CONCLUSIONS: Our data indicated that chronic inflammation, as demonstrated by increased intracellular IL-1 expression, is not associated with increased production of beta-2 microglobulin by monocytes from patients on hemodialysis.

Normal human immunoglobulins for intravenous use (IVIg) delay hyperacute xenograft rejection through F(ab')2-mediated anti-complement activity.

Xenotransplantation between discordant species leads to a hyperacute rejection mediated by natural antibodies, both of the IgG and IgM isotypes, activation of complement and endothelial cell activation. The combination of these mechanisms leads to a transplant survival of minutes to a few hours. Polyclonal human immunoglobulins for intravenous use (IVIg) from normal donors have proved effective in a number of antibody-mediated disorders, as well as in inflammatory disorders. We demonstrate that administration of IVIg in a guinea pig to rat model of cardiac xenografting can effectively delay hyperacute rejection. This effect is mediated by the F(ab')2 fragments of IVIg, and is correlated to an anti-complementary activity.