T cell-dependent immune response in C1q-deficient mice: defective interferon gamma production by antigen-specific T cells

Endogenous complement-activating IgM is not required for primary antibody responses but promotes plasma cell differentiation and secondary antibody responses to a large particulate antigen in mice

Lack of complement factor C1q of the classical pathway results in severely impaired primary antibody responses. This is a paradox because antibodies, especially IgM, are the most efficient activators of the classical pathway and very little specific IgM will be present at priming. A possible explanation would be that natural IgM, binding with low affinity to the antigen, may suffice to activate complement. In support of this, mice lacking secretory IgM have an impaired antibody response, which can be rescued by transfer of non-immune IgM. Moreover, passive administration of specific IgM together with antigen enhances the antibody response in a complement-dependent fashion. To test the idea, we have used a knock-in mouse strain (Cμ13) carrying a point mutation in the IgM heavy chain, rendering the IgM unable to activate complement. Mutant mice backcrossed to BALB/c or C57BL/6 background were primed and boosted with a low dose of sheep red blood cells. Confirming earlier data, no impairment in early, primary IgM- or IgG-responses were seen in either of the Cμ13 strains. However, in one of the mutant strains, late primary IgG responses were impaired. A more pronounced effect was observed after boost, when the IgG response, the number of germinal center B cells and antibody secreting cells as well as the opsonization of antigen were impaired in mutant mice. We conclude that complement activation by natural IgM cannot explain the role of C1q in primary antibody responses, but that endogenous, specific, wildtype IgM generated after immunization feedback-enhances the response to a booster dose of antigen. Importantly, this mechanism can only partially explain the role of complement in the generation of antibody responses because the IgG response was much lower in C3- or complement receptor 1 and 2-deficient mice than in Cμ13 mice.

C1q as a target molecule to treat human disease: What do mouse studies teach us?

The complement system is a field of growing interest for pharmacological intervention. Complement protein C1q, the pattern recognition molecule at the start of the classical pathway of the complement cascade, is a versatile molecule with additional non-canonical actions affecting numerous cellular processes. Based on observations made in patients with hereditary C1q deficiency, C1q is protective against systemic autoimmunity and bacterial infections. Accordingly, C1q deficient mice reproduce this phenotype with susceptibility to autoimmunity and infections. At the same time, beneficial effects of C1q deficiency on disease entities such as neurodegenerative diseases have also been described in murine disease models. This systematic review provides an overview of all currently available literature on the C1q knockout mouse in disease models to identify potential target diseases for treatment strategies focusing on C1q, and discusses potential side-effects when depleting and/or inhibiting C1q.

Yeast-Derived Products: The Role of Hydrolyzed Yeast and Yeast Culture in Poultry Nutrition—A Review

Simple Summary

Yeast and yeast-derived products are largely employed in animal nutrition to support animals’ health and to improve their performance. Thanks to their components, including mannans, β-glucans, nucleotides, vitamins, and other compounds, yeasts have numerous beneficial effects. Among yeast-derived products, hydrolyzed yeasts and yeast cultures have received less attention, but, although the results are somewhat conflicting, in most of the cases, the available literature shows improved performance and health in poultry. Thus, the aim of this review is to provide an overview of hydrolyzed-yeast and yeast-culture employment in poultry nutrition, exploring their effects on the production performance, immune response, oxidative status, gut health, and nutrient digestibility. A brief description of the main yeast bioactive compounds is also provided.

Abstract

Yeasts are single-cell eukaryotic microorganisms that are largely employed in animal nutrition for their beneficial effects, which are owed to their cellular components and bioactive compounds, among which are mannans, β-glucans, nucleotides, mannan oligosaccharides, and others. While the employment of live yeast cells as probiotics in poultry nutrition has already been largely reviewed, less information is available on yeast-derived products, such as hydrolyzed yeast (HY) and yeast culture (YC). The aim of this review is to provide the reader with an overview of the available body of literature on HY and YC and their effects on poultry. A brief description of the main components of the yeast cell that is considered to be responsible for the beneficial effects on animals’ health is also provided. HY and YC appear to have beneficial effects on the poultry growth and production performance, as well as on the immune response and gut health. Most of the beneficial effects of HY and YC have been attributed to their ability to modulate the gut microbiota, stimulating the growth of beneficial bacteria and reducing pathogen colonization. However, there are still many areas to be investigated to better understand and disentangle the effects and mechanisms of action of HY and YC.

CD40-CD154: A perspective from type 2 immunity

The interaction between CD40 and CD154 (CD40 ligand) is central in immunology, participating in CD4⁺ T cell priming by dendritic cells (DC), CD4⁺ T cell help to B cells and classical macrophage activation by CD4⁺ T cells. However, its role in the Th2 side of immunology including helminth infection remains incompletely understood. Contrary to viral and bacterial stimuli, helminth products usually do not cause CD40 up-regulation in DC, and exogenous CD40 ligation drives Th2-biased systems towards Th1. On the other hand, CD40 and CD154 are necessary for induction of most Th2 responses. We attempt to reconcile these observations, mainly by proposing that (i) CD40 up-regulation in DC in Th2 systems is mostly induced by alarmins, (ii) the Th2 to Th1 shift induced by exogenous CD40 ligation is related to the capacity of such ligation to enhance IL-12 production by myeloid cells, and (iii) signals elicited by endogenous CD154 available in Th2 contexts and by exogenous CD40 ligation are probably different. We stress that CD40-CD154 is important beyond cognate cellular interactions. In such a context, we argue that the proliferation response of B-cells to IL-4 plus CD154 reflects a Th2-specific mechanism for polyclonal B-cell amplification and IgE production at infection sites. Finally, we argue that CD154 is a general immune activation signal across immune polarization including Th2, and propose that competition for CD154 at tissue sites may provide negative feedback on response induction at each site.

Glucan and Mannan-Two Peas in a Pod

In recent decades, various polysaccharides isolated from algae, mushrooms, yeast, and higher plants have attracted serious attention in the area of nutrition and medicine. The reasons include their low toxicity, rare negative side effects, relatively low price, and broad spectrum of therapeutic actions. The two most and best-studied polysaccharides are mannan and glucan. This review focused on their biological properties.

Specific IgM and Regulation of Antibody Responses

Specific IgM, administered together with the antigen it recognizes, enhances primary antibody responses, formation of germinal centers, and priming for secondary antibody responses. The response to all epitopes on the antigen to which IgM binds is usually enhanced. IgM preferentially enhances responses to large antigens such as erythrocytes, malaria parasites, and keyhole limpet hemocyanine. In order for an effect to be seen, antigens must be administered in suboptimal concentrations and in close temporal relationship to the IgM. Enhancement is dependent on the ability of IgM to activate complement, but the lytic pathway is not required. Enhancement does not take place in mice lacking complement receptors 1 and 2 (CR1/2) suggesting that the role of IgM is to generate C3 split products, i.e., the ligands for CR1/2. In mice, these receptors are expressed on follicular dendritic cells (FDCs) and B cells. Optimal IgM-mediated enhancement requires that both cell types express CR1/2, but intermediate enhancement is seen when only FDCs express the receptors and low enhancement when only B cells express them. These observations imply that IgM-mediated enhancement works through several, non-mutually exclusive, pathways. Marginal zone B cells can transport IgM-antigen-complement complexes, bound to CR1/2, from the marginal zone and deposit them onto FDCs. In addition, co-crosslinking of the BCR and the CR2/CD19/CD81 co-receptor complex may enhance signaling to specific B cells, a mechanism likely to be involved in induction of early extrafollicular antibody responses.

Polyreactive IgM initiates complement activation by PF4/heparin complexes through the classical pathway

The mechanisms by which exposure to heparin initiates antibody responses in many, if not most, recipients are poorly understood. We recently demonstrated that antigenic platelet factor 4 (PF4)/heparin complexes activate complement in plasma and bind to B cells. Here, we describe how this process is initiated. We observed wide stable variation in complement activation when PF4/heparin was added to plasma of healthy donors, indicating a responder "phenotype" (high, intermediate, or low). Proteomic analysis of plasma from these healthy donors showed a strong correlation between complement activation and plasma immunoglobulin M (IgM) levels (r = 0.898; P < .005), but not other Ig isotypes. Complement activation response to PF4/heparin in plasma displaying the low donor phenotype was enhanced by adding pooled IgM from healthy donors, but not monoclonal IgM. Depletion of IgM from plasma abrogated C3c generation by PF4/heparin. The complement-activating features of IgM are likely mediated by nonimmune, or natural, IgM, as cord blood and a monoclonal polyreactive IgM generate C3c in the presence of PF4/heparin. IgM facilitates complement and antigen deposition on B cells in vitro and in patients receiving heparin. Anti-C1q antibody prevents IgM-mediated complement activation by PF4/heparin complexes, indicating classical pathway involvement. These studies demonstrate that variability in plasma IgM levels correlates with functional complement responses to PF4/heparin. Polyreactive IgM binds PF4/heparin, triggers activation of the classical complement pathway, and promotes antigen and complement deposition on B cells. These studies provide new insights into the evolution of the heparin-induced thrombocytopenia immune response and may provide a biomarker of risk.

Innate immune mediators in cancer: between defense and resistance

Chronic inflammation in the tumor microenvironment and evasion of the antitumor effector immune response are two of the emerging hallmarks required for oncogenesis and cancer progression. The innate immune system not only plays a critical role in perpetuating these tumor-promoting hallmarks but also in developing antitumor adaptive immune responses. Thus, understanding the dual role of the innate system in cancer immunology is required for the design of combined immunotherapy strategies able to tackle established tumors. Here, we review recent advances in the understanding of the role of cell populations and soluble components of the innate immune system in cancer, with a focus on complement, the adapter molecule Stimulator of Interferon Genes, natural killer cells, myeloid cells, and B cells.

IgG3-antigen complexes are deposited on follicular dendritic cells in the presence of C1q and C3

IgG3, passively administered together with small proteins, induces enhanced primary humoral responses against these proteins. We previously found that, within 2 h of immunization, marginal zone (MZ) B cells capture IgG3-antigen complexes and transport them into splenic follicles and that this requires the presence of complement receptors 1 and 2. We have here investigated the localization of IgG3 anti-2, 4, 6-trinitrophenyl (TNP)/biotin-ovalbumin-TNP immune complexes in the follicles and the involvement of classical versus total complement activation in this process. The majority (50-90%) of antigen inside the follicles of mice immunized with IgG3-antigen complexes co-localized with the follicular dendritic cell (FDC) network. Capture of antigen by MZ B cells as well as antigen deposition on FDC was severely impaired in mice lacking C1q or C3, and lack of either C1q or C3 also impaired the ability of IgG3 to enhance antibody responses. Finally, IgG3 efficiently primed for a memory response against small proteins as well as against the large protein keyhole limpet hemocyanine.

IgG Suppresses Antibody Responses in Mice Lacking C1q, C3, Complement Receptors 1 and 2, or IgG Fc-Receptors

Antigen-specific IgG antibodies, passively administered to mice or humans together with large particulate antigens like erythrocytes, can completely suppress the antibody response against the antigen. This is used clinically in Rhesus prophylaxis, where administration of IgG anti-RhD prevents RhD-negative women from becoming immunized against RhD-positive fetal erythrocytes aquired transplacentally. The mechanisms by which IgG suppresses antibody responses are poorly understood. We have here addressed whether complement or Fc-receptors for IgG (FcγRs) are required for IgG-mediated suppression. IgG, specific for sheep red blood cells (SRBC), was administered to mice together with SRBC and the antibody responses analyzed. IgG was able to suppress early IgM- as well as longterm IgG-responses in wildtype mice equally well as in mice lacking FcγRIIB (FcγRIIB knockout mice) or FcγRI, III, and IV (FcRγ knockout mice). Moreover, IgG was able to suppress early IgM responses equally well in mice lacking C1q (C1qA knockout mice), C3 (C3 knockout mice), or complement receptors 1 and 2 (Cr2 knockout mice) as in wildtype mice. Owing to the previously described severely impaired IgG responses in the complement deficient mice, it was difficult to assess whether passively administered IgG further decreased their IgG response. In conclusion, Fc-receptor binding or complement-activation by IgG does not seem to be required for its ability to suppress antibody responses to xenogeneic erythrocytes.

The paradoxical roles of C1q and C3 in autoimmunity

In this review we will focus on the links between complement and autoimmune diseases and will highlight how animal models have provided insights into the manner by which C1q and C3 act to modulate both adaptive and innate immune responses. In particular we will highlight how C1q may not only act as initiator of the classical complement pathway, but can also mediate multiple immune responses in a complement activation independent manner.

Complement protein C1q bound to apoptotic cells suppresses human macrophage and dendritic cell-mediated Th17 and Th1 T cell subset proliferation

A complete genetic deficiency of the complement protein C1q results in SLE with nearly 100% penetrance in humans, but the molecular mechanisms responsible for this association have not yet been fully determined. C1q opsonizes ACs for enhanced ingestion by phagocytes, such as Mϕ and iDCs, avoiding the extracellular release of inflammatory DAMPs upon loss of the membrane integrity of the dying cell. We previously showed that human monocyte-derived Mϕ and DCs ingesting autologous, C1q-bound LALs (C1q-polarized Mϕ and C1q-polarized DCs), enhance the production of anti-inflammatory cytokines, and reduce proinflammatory cytokines relative to Mϕ or DC ingesting LAL alone. Here, we show that C1q-polarized Mϕ have elevated PD-L1 and PD-L2 and suppressed surface CD40, and C1q-polarized DCs have higher surface PD-L2 and less CD86 relative to Mϕ or DC ingesting LAL alone, respectively. In an MLR, C1q-polarized Mϕ reduced allogeneic and autologous Th17 and Th1 subset proliferation and demonstrated a trend toward increased Treg proliferation relative to Mϕ ingesting LAL alone. Moreover, relative to DC ingesting AC in the absence of C1q, C1q-polarized DCs decreased autologous Th17 and Th1 proliferation. These data demonstrate that a functional consequence of C1q-polarized Mϕ and DC is the regulation of Teff activation, thereby "sculpting" the adaptive immune system to avoid autoimmunity, while clearing dying cells. It is noteworthy that these studies identify novel target pathways for therapeutic intervention in SLE and other autoimmune diseases.

Complement modulation of T cell immune responses during homeostasis and disease

The complement system is an ancient and critical effector mechanism of the innate immune system as it senses, kills, and clears infectious and/or dangerous particles and alerts the immune system to the presence of the infection and/or danger. Interestingly, an increasing number of reports have demonstrated a clear role for complement in the adaptive immune system as well. Of note, a number of recent studies have identified previously unknown roles for complement proteins, receptors, and regulators in T cell function. Here, we will review recent data demonstrating the influence of complement proteins C1q, C3b/iC3b, C3a (and C3aR), and C5a (and C5aR) and complement regulators DAF (CD55) and CD46 (MCP) on T cell function during homeostasis and disease. Although new concepts are beginning to emerge in the field of complement regulation of T cell function, future experiments should focus on whether complement is interacting directly with the T cell or is having an indirect effect on T cell function via APCs, the cytokine milieu, or downstream complement activation products. Importantly, the identification of the pivotal molecular pathways in the human systems will be beneficial in the translation of concepts derived from model systems to therapeutic targeting for treatment of human disorders.

How antibodies use complement to regulate antibody responses

Antibodies, forming immune complexes with their specific antigen, can cause complete suppression or several 100-fold enhancement of the antibody response. Immune complexes containing IgG and IgM may activate complement and in such situations also complement components will be part of the immune complex. Here, we review experimental data on how antibodies via the complement system upregulate specific antibody responses. Current data suggest that murine IgG1, IgG2a, and IgG2b upregulate antibody responses primarily via Fc-receptors and not via complement. In contrast, IgM and IgG3 act via complement and require the presence of complement receptors 1 and 2 (CR1/2) expressed on both B cells and follicular dendritic cells. Complement plays a crucial role for antibody responses not only to antigen complexed to antibodies, but also to antigen administered alone. Lack of C1q, but not of Factor B or MBL, severely impairs antibody responses suggesting involvement of the classical pathway. In spite of this, normal antibody responses are found in mice lacking several activators of the classical pathway (complement activating natural IgM, serum amyloid P component (SAP), specific intracellular adhesion molecule-grabbing non-integrin R1 (SIGN-R1) or C-reactive protein. Possible explanations to these observations will be discussed.

Complement-activating IgM enhances the humoral but not the T cell immune response in mice

IgM antibodies specific for a certain antigen can enhance antibody responses when administered together with this antigen, a process believed to require complement activation by IgM. However, recent data show that a knock-in mouse strain, Cμ13, which only produces IgM unable to activate complement, has normal antibody responses. Moreover, the recently discovered murine IgM Fc receptor (FcµR or TOSO/FAIM3) was shown to affect antibody responses. This prompted the re-investigation of whether complement activation by specific IgM is indeed required for enhancement of antibody responses and whether the mutation in Cµ13 IgM also caused impaired binding to FcµR. The results show that IgM from Cµ13 and wildtype mice bound equally well to the murine FcµR. In spite of this, specific Cμ13 IgM administered together with sheep red blood cells or keyhole limpet hemocyanine was a very poor enhancer of the antibody and germinal center responses as compared with wildtype IgM. Within seconds after immunization, wildtype IgM induced deposition of C3 on sheep red blood cells in the blood. IgM which efficiently enhanced the T-dependent humoral immune response had no effect on activation of specific CD4⁺ T cells as measured by cell numbers, cell division, blast transformation, or expression of the activation markers LFA-1 and CD44 in vivo. These observations confirm the importance of complement for the ability of specific IgM to enhance antibody responses and suggest that there is a divergence between the regulation of T- and B-cell responses by IgM.

Complement receptors 1 and 2 in murine antibody responses to IgM-complexed and uncomplexed sheep erythrocytes

Early complement components are important for normal antibody responses. In this process, complement receptors 1 and 2 (CR1/2), expressed on B cells and follicular dendritic cells (FDCs) in mice, play a central role. Complement-activating IgM administered with the antigen it is specific for, enhances the antibody response to this antigen. Here, bone marrow chimeras between Cr2^−/− and wildtype mice were used to analyze whether FDCs or B cells must express CR1/2 for antibody responses to sheep erythrocytes (SRBC), either administered alone or together with specific IgM. For robust IgG anti-SRBC responses, CR1/2 must be expressed on FDCs. Occasionally, weak antibody responses were seen when only B cells expressed CR1/2, probably reflecting extrafollicular antibody production enabled by co-crosslinking of CR2/CD19/CD81 and the BCR. When SRBC alone was administered to mice with CR1/2⁺ FDCs, B cells from wildtype and Cr2^−/− mice produced equal amounts of antibodies. Most likely antigen is then deposited on FDCs in a way that optimizes engagement of the B cell receptor, making CR2-facilitated signaling to the B cell superfluous. SRBC bound to IgM will have more C3 fragments, the ligands for CR1/2, on their surface than SRBC administered alone. Specific IgM, forming a complex with SRBC, enhances antibody responses in two ways when FDCs express CR1/2. One is dependent on CR1/2⁺ B cells and probably acts via increased transport of IgM-SRBC-complement complexes bound to CR1/2 on marginal zone B cells. The other is independent on CR1/2⁺ B cells and the likely mechanism is that IgM-SRBC-complement complexes bind better to FDCs than SRBC administered alone. These observations suggest that the immune system uses three different CR1/2-mediated effector functions to generate optimal antibody responses: capture by FDCs (playing a dominant role), transport by marginal zone B cells and enhanced B cell signaling.

Requirement for complement in antibody responses is not explained by the classic pathway activator IgM

Animals lacking complement factors C1q, C2, C3, or C4 have severely impaired Ab responses, suggesting a major role for the classic pathway. The classic pathway is primarily initiated by antigen-Ab complexes. Therefore, its role for primary Ab responses seems paradoxical because only low amounts of specific Abs are present in naive animals. A possible explanation could be that the classic pathway is initiated by IgM from naive mice, binding with sufficient avidity to the antigen. To test this hypothesis, a knock-in mouse strain, Cμ13, with a point mutation in the gene encoding the third constant domain of the μ-heavy chain was constructed. These mice produce IgM in which proline in position 436 is substituted with serine, a mutation previously shown to abrogate the ability of mouse IgM to activate complement. Unexpectedly, the Ab response to sheep erythrocytes and keyhole limpet hemocyanin in Cμ13 mice was similar to that in WT mice. Thus, although secreted IgM and the classic pathway activation are both required for the normal primary Ab response, this does not require that IgM activate C. This led us to test Ab responses in animals lacking one of three other endogenous activators of the classic pathway: specific intracellular adhesion molecule-grabbing nonintegrin R1, serum amyloid P component, and C-reactive protein. Ab responses were also normal in these animals.

Complement and non-complement activating functions of C1q: A prototypical innate immune molecule

C1q is a versatile innate immune molecule that serves as the initiation subcomponent of the classical complement pathway. In addition, it is also a potent pattern recognition molecule, the versatility of which has fuelled its functional flexibility. C1q recognises an array of self, non-self and altered-self ligands. The broad-spectrum ligand-binding potential of C1q is facilitated by the modular organisation of the heterotrimeric globular head region, its ability to change its conformation in a very subtle way, and the manner in which this ancient molecule appears to have evolved to deal with the different types of ligands. Over recent years, molecules that resemble C1q have been put together to form the C1q family. In this review, we briefly summarise complement-dependent and complement-independent functions of C1q, its cognate receptors and key members of the rapidly growing C1q family.

C1q regulation of dendritic cell development from monocytes with distinct cytokine production and T cell stimulation

The causative association of complement C1q deficiency with systemic lupus erythematosus (SLE), which inevitably involves the breakdown of tolerance, remains poorly explained. Its non-hepatic, macrophage and dendritic cell (DC) origin may be highly relevant. In tissues, C1q is produced by DCs and macrophages which deposits around these cells and we ask whether this pericellular form of C1q regulates DC development from monocytes. DCs cultured on immobilized C1q (C1q-DCs) show similar MHC, CD40, CD80, CD86, CD83 and CCR7 expression as normal DCs, but these cells exhibit increased phagocytosis of apoptotic cells and elevated IL-10 but reduced IL-12 and IL-23 production. Intracellularly, C1q-DCs exhibit increased ERK, p38 and p70S6 kinase activity. By mixed leukocyte reaction, C1q-DCs show reduced Th1 and Th17 induction from allogeneic CD4(+) T cells. LPS and IFNγ, which cause normal DCs to induce increased CD25 expression on CD4(+) T cells, attenuate C1q-DC induction of CD25. These imply that the DC pericellular C1q may induce tolerogenic properties in developing DCs.

The non-classical functions of the classical complement pathway recognition subcomponent C1q

C1q, the ligand recognition subcomponent of the classical complement pathway has steadily been gaining recognition as a bridge between innate and adaptive immunity. C1q has been shown to be involved in the modulation of various immune cells (such as dendritic cells, platelets, microglia cells and lymphocytes), clearance of apoptotic cells, a range of cell processes such as differentiation, chemotaxis, aggregation and adhesion, and pathogenesis of neurodegenerative diseases and systemic lupus erythematosus. Recent studies have highlighted the importance of C1q during pregnancy, coagulation process and embryonic development including neurological synapse function. It is intriguing to note that a prototypical defence molecule has so many diverse functions that probably have its origin in its versatility as a potent charge pattern recognition molecule, modularity within the ligand-recognising globular domain, and the redundancy of putative C1q receptors. The range of function that C1q has been shown to perform also provides clues for the undiscovered functions of a number of C1q family members.

Complement and its role in protection and pathogenesis of flavivirus infections

The complement system is a family of serum and cell surface proteins that recognize pathogen-associated molecular patterns, altered-self ligands, and immune complexes. Activation of the complement cascade triggers several antiviral functions including pathogen opsonization and/or lysis, and priming of adaptive immune responses. In this review, we will examine the role of complement activation in protection and/or pathogenesis against infection by Flaviviruses, with an emphasis on experiments with West Nile and Dengue viruses.

C1q enhances IFN-gamma production by antigen-specific T cells via the CD40 costimulatory pathway on dendritic cells

Dendritic cells (DCs) are known to produce C1q, the initiator of the classical complement pathway. We demonstrate that murine DCs deficient in C1q (C1qa(-/-)) are poorer than wild-type (WT) DCs at eliciting the proliferation and Th1 differentiation of antigen-specific T cells. These defects result from decreased production of IL-12p70 by C1qa(-/-) DCs and impaired expression of costimulatory molecules CD80 and CD86 in response to CD40 ligation. The defective production of IL-12p70 and the reduced expression of CD80 and CD86 by C1qa(-/-) DCs were specifically mediated via CD40 ligation, as normal levels of IL-12p70 and CD80/86 were observed after ligation of Toll-like receptors (TLRs) on C1qa(-/-) DCs. CD40 ligation on C1qa(-/-) DCs, but not TLR ligation, results in decreased phosphorylation of p38 and ERK1/2 kinases. A strong colocalization of CD40 and C1q was observed by confocal microscopy upon CD40 ligation (but not TLR ligation) on DCs. Furthermore, human DCs from 2 C1q-deficient patients were found to have impaired IL-12p70 production in response to CD40L stimulation. Our novel data suggest that C1q augments the production of IL-12p70 by mouse and human DCs after CD40 triggering and plays important roles in sustaining the maturation of DCs and guiding the activation of T cells.

Complement production and regulation by dendritic cells: molecular switches between tolerance and immunity

In recent years it has become clear that the innate and adaptive immune systems are highly integrated and interact at several levels. Dendritic cells (DCs) are on the one hand instrumental for directing and controlling adaptive immunity and on the other hand are specialized in detecting and integrating signals from the microenvironment. In view of the strong link between deficiencies in certain complement components and the development of autoimmunity, interaction between complement and DCs seems to be of fundamental importance. We will discuss the role of C1q, C3, as well as complement regulators in DC biology.

Food restriction compromises immune memory in deer mice (Peromyscus maniculatus) by reducing spleen-derived antibody-producing B cell numbers

Immune activity is variable in many wild animals, despite presumed strong selection against immune incompetence. Much variation may be due to changes in prevalence and abundance of pathogens (and/or their vectors) in time and space, but the costs of immune defenses themselves may also be important. Induction of immune activity often increases energy and protein expenditure, sometimes to the point of compromising fitness. Whether immune defenses are expensive to maintain once they are generated, however, is less well appreciated. If so, organisms would face persistent challenges of allocating resources between immunity and other expensive physiological processes, which would mandate trade-offs. Mild food restriction (70% ad lib. diet) reduces secondary antibody responses in deer mice (Peromyscus maniculatus), functionally representing a cost of immune memory. In this study, we asked whether compromised immune memory was mediated by a decrease in size of the B cell population responsible for producing antibodies (i.e., spleen-derived B lymphocytes producing immunoglobulin G [IgG]). Two weeks of food restriction reduced total splenocytes, total splenic B lymphocytes (B220+ cells), and splenic B lymphocytes producing IgG (B220+/IgG+ cells) but did not affect body mass or two circulating antibody subclasses (IgG1 and IgG2a) in deer mice. These results further indicate that maintenance of immune memory is expensive and may be subject to trade-offs with other physiological processes.

The classical complement pathway in transplantation: unanticipated protective effects of C1q and role in inductive antibody therapy

Though complement (C) deposition within the transplant is associated with allograft rejection, the pathways employed have not been established. In addition, evidence suggests that C-mediated cytolysis may be necessary for the tolerance-inducing activities of mAb therapies. Hence, we assessed the role of the classical C pathway in acute allograft rejection and its requirement for experimental mAb therapies. C1q-deficient (C1q-/-) recipients rejected allografts at a faster rate than wild-type (WT) recipients. This rejection was associated with exacerbated graft pathology but not with enhanced T-cell responses in C1q-/- recipients. However, the humoral response to donor alloantigens was accelerated in C1q-/- mice, as an early IgG response and IgG deposition within the graft were observed. Furthermore, deposition of C3d, but not C4d was observed in grafts isolated from C1q-/- recipients. To assess the role of the classical C pathway in inductive mAb therapies, C1q-/- recipients were treated with anti-CD4 or anti-CD40L mAb. The protective effects of anti-CD4 mAb were reduced in C1q-/- recipients, however, this effect did not correlate with ineffective depletion of CD4+ cells. In contrast, the protective effects of anti-CD40L mAb were less compromised in C1q-/- recipients. Hence, this study reveals unanticipated roles for C1q in the rejection process.

The role of complement in the success of vaccination with conjugated vs. unconjugated polysaccharide antigen

The complement system, a well-characterised arm of the innate immune system, significantly influences the adaptive immune response via direct cell-cell interaction and maintenance of lymphoid organ architecture. Development of vaccines is a major advance in modern health care. In this review, we highlight the importance of the marginal zone in response to both, polysaccharide and conjugated vaccines, and discuss the relevance of complement herein, based on findings obtained from animal models with specific deletions of certain complement components and from vaccination reports of complement-deficient individuals. We conclude that both, intactness of the complement system and maturity of expression of its components, are relatively more important to aid in the immune response to polysaccharide vaccine than to conjugated vaccines.

The regulatory roles of C1q

C1q binds to immune complexes to elicit complement-dependent microbial killing and enhance phagocytosis. Besides this classical role, C1q also opsonizes apoptotic cells for clearance by phagocytes. C1q deficiency increases susceptibility to microbial infections and is also associated with elevated autoimmunity as characterized by increased apoptotic bodies in tissues. Most complement proteins are of liver origin, but C1q is predominantly synthesized by peripheral tissue macrophages and dendritic cells. Besides being found in the blood, C1q has also been found deposited in extracellular tissues around these cells. In vitro, immobilized C1q inhibits monocyte, macrophage and T-cell production of inflammatory cytokines. It also regulates T-cell activation. Therefore, mounting evidence suggest a major regulatory role for C1q in inflammation and autoimmunity.

Mannose-binding lectin A-deficient mice have abrogated antigen-specific IgM responses and increased susceptibility to a nematode infection

To investigate the role of mannose-binding lectin-A (MBL-A) in protection against infectious disease, MBL-A(-/-)-deficient mice were generated. Using a well-characterized mouse model of human filarial nematode infection, nematode survival and protective immune responses were tested in vivo. Blood-borne Brugia malayi microfilariae survived for significantly longer time periods in MBL-A(-/-) than in wild-type (WT) mice. However, no differences in either splenic cytokine responses or induction of leukocytes in the blood were observed. A profound abrogation of Ag-specific IgM levels was measured in B. malayi-infected MBL-A(-/-) mice, and some IgG isotypes were higher than those observed in WT animals. To establish whether there was a defect in Ab responses per se in MBL-A(-/-) mice or the effect was specific to filarial infection, we immunized these mice with OVA or a carbohydrate-free protein. Significantly, Ag-specific IgM responses were defective to both of these Ags, and Ag-specific IgG responses were largely unaffected. Furthermore, in naive mice, total IgM levels did not differ between MBL-A(-/-) and WT mice. This article describes the first demonstration that MBL-A may function independently of MBL-C and suggests that MBL-A, like other C-type lectins and members of the complement cascade, is intimately involved in the priming of the humoral Ab response.

Antibody-independent B cell-intrinsic and -extrinsic roles for CD21/35

Mice lacking C3, C4 or complement receptor 1/2 (Cr) have defective germinal centers (GC). The requirement for C4 implicates complement fixation by immune complexes (IC) via the classical pathway. Yet, transgenic (Tg) mice that lack circulating antibody but still express membrane IgM (mIgM) have normal GC responses. We showed previously that cross-linking mIgM leads to the deposition of C3 on the B cell surface and that disruption of this pathway diminishes GC responses. Here, we investigate the role of Cr in this process by generating mIgM-Tg mice that lack Cr and serum Ig. These mIgM/Cr-/- mice have smaller, transient GC, with incomplete B cell receptor down-regulation and peanut agglutinin up-regulation, compared to mIgM/Crwt counterparts. BM chimera experiments showed that Cr on B cells is required for normal GC responses. These results establish that Cr ligands generated at the B cell surface are sufficient for normal GC responses and function by signaling Cr on B cells. Unexpectedly, chimera experiments also showed a critical role for Cr on follicular dendritic cells (FDC), even in the absence of IC, indicating novel functions for FDC-expressed Cr beyond the capture of C3-coated IC.

Antibody-mediated regulation of the immune response

Antibodies administered in vivo together with the antigen they are specific for can regulate the immune response to that antigen. This phenomenon is called antibody-mediated feedback regulation and has been known for over 100 years. Both passively administered and actively produced antibodies exert immunoregulatory functions. Feedback regulation can be either positive or negative, resulting in >1000-fold enhancement or >99% suppression of the specific antibody response. Usually, the response to the entire antigen is up- or downregulated, regardless of which epitope the regulating antibody recognizes. IgG of all isotypes can suppress responses to large particulate antigens like erythrocytes, a phenomenon used clinically in Rhesus prophylaxis. IgG suppression works in mice lacking the known Fc-gamma receptors (FcgammaR) and a likely mechanism of action is epitope masking. IgG1, IgG2a and IgG2b administered together with soluble protein antigens will enhance antibody and CD4+ T-cell responses via activating FcgammaR, probably via increased antigen presentation by dendritic cells. IgG3 as well as IgM also enhance antibody responses but their effects are dependent on their ability to activate complement. A possible mechanism is increased B-cell activation caused by immune complexes co-crosslinking the B-cell receptor with the complement-receptor 2/CD19 receptor complex, known to lower the threshold for B-cell activation. IgE-antibodies enhance antibody and CD4+ T-cell responses to small soluble proteins. This effect is entirely dependent on the low-affinity receptor for IgE, CD23, the mechanism probably being increased antigen presentation by CD23+ B cells.

A Novel Mechanism for Complement Activation at the Surface of B Cells Following Antigen Binding

Coligation of CD21 with BCR on the surface of B cells provides a costimulatory signal essential for efficient Ab responses to T-dependent Ags. To achieve this, Ag must be directly linked to C3 fragments, but how this occurs in vivo is not fully understood. Using BCR transgenic mice, we demonstrated that C3 was deposited on the surface of B cells following both high- and moderate-affinity Ag binding. This was dependent on the specific binding of IgM to the BCR-bound Ag and can occur independently of soluble immune complex formation. Based on these data, we propose a novel model in which immune complexes can form directly on the surface of the B cell following Ag binding. This model has implications for our understanding of B lymphocyte activation.

Complement 3d: from molecular adjuvant to target of immune escape mechanisms

C3d is a fragment of the complement factor C3 and is generated in the course of complement activation. When bound to antigen in single or multiple copies, the B cell receptor and complement receptor 2 become co-crosslinked resulting in decreased or increased B cell responses depending on the valence of the antigen-C3d construct. When antigen-C3d constructs are used for the purpose of generating a protective immune response (vaccines), they may either enhance the expected response or suppress it depending on the nature of the antigen. Various pathogens use C3d to evade the immune system by inhibiting complement activation, invading and homing in host cells or masking immunogenic areas of pathogen proteins. Therefore, future vaccination strategies for infectious diseases and cancer employing C3d as a molecular adjuvant need to be carefully evaluated before choosing a target antigen in order to take advantage of the adjuvant effect of the complement component while avoiding potential vaccine complications associated with immune escape mechanisms.

Protective immune responses against West Nile virus are primed by distinct complement activation pathways

West Nile virus (WNV) causes a severe infection of the central nervous system in several vertebrate animals including humans. Prior studies have shown that complement plays a critical role in controlling WNV infection in complement (C) 3(-/-) and complement receptor 1/2(-/-) mice. Here, we dissect the contributions of the individual complement activation pathways to the protection from WNV disease. Genetic deficiencies in C1q, C4, factor B, or factor D all resulted in increased mortality in mice, suggesting that all activation pathways function together to limit WNV spread. In the absence of alternative pathway complement activation, WNV disseminated into the central nervous system at earlier times and was associated with reduced CD8+ T cell responses yet near normal anti-WNV antibody profiles. Animals lacking the classical and lectin pathways had deficits in both B and T cell responses to WNV. Finally, and somewhat surprisingly, C1q was required for productive infection in the spleen but not for development of adaptive immune responses after WNV infection. Our results suggest that individual pathways of complement activation control WNV infection by priming adaptive immune responses through distinct mechanisms.

Structural and functional anatomy of the globular domain of complement protein C1q

C1q is the first subcomponent of the classical pathway of the complement system and a major connecting link between innate and acquired immunity. As a versatile charge pattern recognition molecule, C1q is capable of engaging a broad range of ligands via its heterotrimeric globular domain (gC1q) which is composed of the C-terminal regions of its A (ghA), B (ghB) and C (ghC) chains. Recent studies using recombinant forms of ghA, ghB and ghC have suggested that the gC1q domain has a modular organization and each chain can have differential ligand specificity. The crystal structure of the gC1q, molecular modeling and protein engineering studies have combined to illustrate how modular organization, charge distribution and the spatial orientation of the heterotrimeric assembly offer versatility of ligand recognition to C1q. Although the biochemical and structural studies have provided novel insights into the structure-function relationships within the gC1q domain, they have also raised many unexpected issues for debate.

Cord blood and adult T cells show different responses to C1q-bearing immune complexes

We have previously shown that T cells can be activated through cell-surface C1q receptors, resulting in secretion of interferon-gamma (IFN-gamma) and tumor necrosis factor alpha (TNFalpha), further demonstrating the intimate linkage between innate and adaptive immunity. In this current report, we sought to determine whether: (1) T cell responses to C1q-bearing immune complexes are dependent on the maturational status of the T cells and (2) whether signaling through the C1q receptor on T cells modulates conventional activation mediated through the conventional T cell receptor (TCR)/CD3 signaling complex. We first examined the capacity of neonatal T cells to respond to C1q-bearing immune complexes using IFNgamma, IL-2, and MIF secretion as measures of activation (MIF was chosen because of its crucial role in coordinating innate and adaptive immunity). Neonatal T cells produced significantly less IFNgamma but not IL-2, when stimulated by C1q immune complexes compared with adult T cells. MIF levels did not exceed background levels in these experiments. Next, we examined the capacity of C1q-bearing immune complexes to regulate signaling through the conventional TCR/CD3 signaling complex. Pre-incubating adult T cells with C1q-bearing immune complexes significantly reduced IFNgamma secretion when those same cells were subsequently stimulated with anti-CD3 and anti-CD28 monoclonal antibodies. Pre-incubation of neonatal T-cells with C1q-bearing immune complexes had no effect on IFNgamma secretion, although IFNgamma secretion was lower than that found in adult T cells for each experimental condition. We speculate that reduced IFNgamma secretion after pre-incubation with C1q immune complexes may be due to IL-10 secretion, which was observed in C1q-stimulated adult (but not neonatal) T cells.

CONCLUSIONS

C1q-bearing immune complexes exert complex effects on mature T cells that include both pro- and anti-inflammatory responses. Immunologic maturation is required for these effects, as cord blood T cells are relatively hyporesponsive to C1q-bearing immune complexes compared with adult T cells.

The complement system in regulation of adaptive immunity

The serum complement system, which represents a chief component of innate immunity, not only participates in inflammation but also acts to enhance the adaptive immune response. Specific activation of complement via innate recognition proteins or secreted antibody releases cleavage products that interact with a wide range of cell surface receptors found on myeloid, lymphoid and stromal cells. This intricate interaction among complement activation products and cell surface receptors provides a basis for the regulation of both B and T cell responses. This review highlights fundamental events, explaining how complement links innate and adaptive immunity as well as describing more recent studies on how this large family of proteins functions locally in peripheral lymph nodes to enhance B and T cell responses.

A protective role for innate immunity in systemic lupus erythematosus

Clinical and genetic studies in humans and animal models indicate a crucial protective role for the complement system in systemic lupus erythematosus (SLE). This presents a paradox because the complement system is considered to be an important mediator of the inflammation that is observed in patients with SLE. One current view is that complement provides protection by facilitating the rapid removal of apoptotic debris to circumvent an autoimmune response. In this Opinion article, I discuss an alternative model in which complement - together with other components of the innate immune system - participates in the 'presentation' of SLE-inducing self-antigens to developing B cells. In this way, the complement system and innate immunity protect against responses to SLE (self) antigens by enhancing the elimination of self-reactive lymphocytes.

The role of complement in the development of systemic lupus erythematosus

Complement has both beneficial and deleterious roles in the pathogenesis of systemic lupus erythematosus (SLE). On the one hand, patients with SLE present with decreased complement levels and with complement deposition in inflamed tissues, suggestive of a harmful role of complement in the effector phase of disease. On the other hand, homozygous deficiency of any of the classical pathway proteins is strongly associated with the development of SLE. There are two main hypotheses to explain these observations. The first invokes an important role for complement in the physiological waste-disposal mechanisms of dying cells and immune complexes. The second hypothesis is based around the role of complement in determining the activation thresholds of B and T lymphocytes, with the proposal that complement deficiency causes incomplete maintenance of peripheral tolerance. These two hypotheses are not mutually exclusive. In addition, there is evidence for a contribution from other genetic factors in determining the phenotype of disease in the absence of complement.

Myeloid C3 determines induction of humoral responses to peripheral herpes simplex virus infection

The complement system, in addition to its role in innate immunity, is an important regulator of the B cell response. Complement exists predominantly in the circulation and although the primary source is hepatic, multiple additional cellular sources have been described that can contribute substantially to the complement pool. To date, however, complement produced by these secondary sources has been deemed redundant to that secreted by the liver. In contrast, using a bone marrow chimeric model, we observed that C3 synthesis by myeloid cells, a relatively minor source of complement, provided a critical function during the induction of humoral responses to peripheral HSV infection. Anti-viral Ab, as generated in an efficient humoral response, has been associated with protection from severe consequences of HSV dissemination. This report offers insight into the generation of the adaptive immune response in the periphery and describes a unique role for a nonhepatic complement source.

Innate and adaptive immune responses determine protection against disseminated infection by West Nile encephalitis virus

WNV continues to spread throughout the Western Hemisphere as virus activity in insects and animals has been reported in the United States, Canada, Mexico, and the Caribbean islands. West Nile virus (WNV) infects the central nervous system and causes severe disease primarily in humans who are immunocompromised or elderly. In this review, we discuss the mechanisms by which the immune system limits dissemination of WNV infection. Recent experimental studies in animals suggest important roles for both the innate and the adaptive immune responses in controlling WNV infection. Interferons, antibody, complement components and CD8+ T cells coordinate protection against severe infection and disease. These findings are analyzed in the context of recent approaches to vaccine development and immunotherapy against WNV.

Autoimmune Renal Injury in C3- and C4-Deficient Mice: A Histological and Functional Study

BACKGROUND

Complement deficiency predisposes to autoimmune renal disease. Since complement deficient mice are increasingly used to study the immunopathogenesis of renal disease we have determined whether mice deficient in C3 or C4 are susceptible to spontaneous immune-mediated renal injury.

METHODS

C3-deficient, C4-deficient and complement-sufficient, wild-type mice were maintained in standard conditions for 1 year at which stage renal function, renal histology, circulating antibody and autoantibody levels were assessed.

RESULTS

No significant decline in renal function was demonstrated in the complement-deficient mice. However, there was histological evidence of glomerular injury in both the C3- and C4-deficient mice, but of insufficient severity to alter function. Serum IgG2a concentration was significantly lower in C3- and C4-deficient mice. In contrast C3-deficient mice had higher concentrations of serum IgG2b. There was a tendency for mice from all groups, including the complement-sufficient mice, to develop autoantibodies. C4-deficient mice had higher titres of anti-dsDNA IgG but otherwise deficient mice had similar autoantibody titres to controls.

CONCLUSION

We conclude that C4-deficient mice demonstrate a small increase in autoantibody production at 1 year of age compared to C3-deficient and wild-type mice. Furthermore, although complement-deficient mice exhibit glomerular changes, they are of minor functional significance, and are unlikely to affect the study of experimentally induced renal disease in these mice.

The B cell receptor itself can activate complement to provide the complement receptor 1/2 ligand required to enhance B cell immune responses in vivo

B cells express complement receptors (CRs) that bind activated fragments of C3 and C4. Immunized CR knockout (KO) mice have lower antibody titers and smaller germinal centers (GCs), demonstrating the importance of CR signals for the humoral immune response. CR ligands were thought to be generated via complement fixation mediated by preexisting "natural" IgM or early Ab from inefficiently activated B cells. This concept was recently challenged by a transgenic (Tg) mouse model that lacks circulating antibody but still retains membrane IgM (mIgM) and mounts normal immune responses. To test whether CR ligands could be generated by the B cell receptor (BCR) itself, we generated similar mice carrying a mutated mIgM that was defective in C1q binding. We found that B cells from such mutant mice do not deposit C3 on B cells upon BCR ligation, in contrast to B cells from mIgM mice. This has implications for the immune response: the mutant mice have smaller GCs than mIgM mice, and they are particularly deficient in the maintenance of the GC response. These results demonstrate a new BCR-dependent pathway that is sufficient and perhaps necessary to provide a CR1/2 ligand that promotes efficient B cell activation.

Immunologically induced, complement-dependent up-regulation of the prion protein in the mouse spleen: follicular dendritic cells versus capsule and trabeculae

The expression of the prion protein (PrP) in the follicular dendritic cell network of germinal centers in the spleen is critical for the splenic propagation of the causative agent of prion diseases. However, a physiological role of the prion protein in the periphery remains elusive. To investigate the role and function of PrP expression in the lymphoid system we treated naive mice i.v. with preformed immune complexes or vesicular stomatitis virus. Immunohistochemistry and Western blot analysis of the spleen revealed that 8 days after immunization, immune complexes and vesicular stomatitis virus had both induced a strong increase of PrP expression in the follicular dendritic cell network. Remarkably, this up-regulation did not occur in mice that lack an early factor of the complement cascade, C1q, a component which has been shown previously to facilitate early prion pathogenesis. In addition to the variable PrP level in the germinal centers, we detected steady and abundant PrP expression in the splenic capsule and trabeculae, which are structural elements that have not been associated before with PrP localization. The abundant trabeculo-capsular PrP expression was also evident in spleens of Rag-1-deficient mice, which have been shown before to be incapable of prion expansion. We conclude that trabeculocapsular PrP is not sufficient for splenic prion propagation. Furthermore, our observations may provide important clues for a physiological function of the prion protein and allow a new view on the role of complement and PrP in peripheral prion pathogenesis.

Antigen-capturing cells can masquerade as memory B cells

As well as classically defined switched immunoglobulin isotype-expressing B cells, memory B cells are now thought to include IgM-expressing cells and memory cells that lack B cell lineage markers, such as B220 or CD19. We set out to compare the relative importance of memory B cell subsets with an established flow cytometry method to identify antigen-specific cells. After immunization with PE, we could detect B220+ and, as reported previously, B220- antigen-binding cells (McHeyzer-Williams, L.J., M. Cool, and M.G. McHeyzer-Williams. 2001. J. Immunol. 167:1393-1405). The B220-PE+ cells bore few markers typical of B cells, but resembled myeloid cells. Further analysis of the antigen-binding characteristics of these cells showed that, upon immunization with two fluorescent proteins, the B220- cells could bind both. Furthermore, this subpopulation was detected in RAG1-/- mice after transfer of anti-PE mouse serum. These data strongly suggest that these cells capture serum Ig, via Fc receptors, and thus appear antigen-specific. Investigation of these antigen-capturing cells in a variety of knockout mice indicates that they bind monomeric IgG in an FcgammaR1 (CD64)-dependent manner. We find no evidence of a B220- memory B cell population that is not explicable by antigen-capturing cells, and warn that care must be taken when using antigen-specificity or surface IgG as an indicator of B cell memory.

In vivo biosynthesis of endogenous and of human C1 inhibitor in transgenic mice: tissue distribution and colocalization of their expression

We have produced transgenic mice expressing human C1 inhibitor mRNA and protein under the control of the human promoter and regulatory elements. The transgene was generated using a minigene construct in which most of the human C1 inhibitor gene (C1NH) was replaced by C1 inhibitor cDNA. The construct retained the promoter region extending 1.18 kb upstream of the transcription start site, introns 1 and 2 as well as a stretch of 2.5 kb downstream of the polyadenylation site, and therefore carried all known elements involved in transcriptional regulation of the C1NH gene. Mice with high serum levels of human C1 inhibitor, resulting from multiple tandem integrations of the C1 inhibitor transgene, were selected. Immunohistochemistry in combination with in situ hybridization was applied to localize the sites of C1 inhibitor biosynthesis and to demonstrate its local production in brain, spleen, liver, heart, kidney, and lung. The distribution of human C1 inhibitor-expressing cells was qualitatively indistinguishable from that of its mouse counterpart, but expression levels of the transgene were significantly higher. In the spleen, production of C1 inhibitor was colocalized with that of a specific marker for white pulp follicular dendritic cells. This study demonstrates a stringently regulated expression of both the endogenous and the transgenic human C1 inhibitor gene and reveals local biosynthesis of C1 inhibitor at multiple sites in which the components of the macromolecular C1 complex are also produced.

Macrophage-derived complement component C4 can restore humoral immunity in C4-deficient mice

Mice with a disrupted C4 locus (C4(-/-)) have an impaired immune response to thymus-dependent Ags. To test the role of bone marrow-derived C4 in humoral immunity, we reconstituted deficient animals with wild-type bone marrow or an enriched fraction of bone marrow-derived macrophages. C4 chimeras were immunized with 4-hydroxy-3-nitrophenyl(5) conjugated to keyhole limpet hemocyanin (NP(5)- KLH) or infected with HSV-1, and the Ab response was evaluated. Wild-type bone marrow rescued the humoral immune response to both Ags, i.e., the soluble Ag and HSV-1, demonstrating that local C4 production is sufficient for humoral responses. Although the C4 chimeric animals lacked detectable C4 in their sera, C4 mRNA was identified in splenic sections by in situ hybridization, and C4 protein deposits were identified in the germinal center areas of splenic follicles by immunofluorescence staining. Macrophages derived from bone marrow produced sufficient C4 protein to restore the humoral response to NP(5)-KLH in C4-deficient animals when administered along with Ag. Cell-sorting experiments, followed by C4-specific RT-PCR, identified splenic macrophages (CD11b(+), CD11c(-)) as a cellular source for C4 synthesis within the spleen.