Multiplex screening for functionally rearranged immunoglobulin variable regions reveals expression of hybridoma-specific aberrant V-genes

Determination of variable region sequences from hybridoma immunoglobulins that target Mycobacterium tuberculosis virulence factors

Mycobacterium tuberculosis (Mtb) infects one-quarter of the world's population. Mtb and HIV coinfections enhance the comorbidity of tuberculosis (TB) and AIDS, accounting for one-third of all AIDS-associated mortalities. Humoral antibody to Mtb correlates with TB susceptibility, and engineering of Mtb antibodies may lead to new diagnostics and therapeutics. The characterization and validation of functional immunoglobulin (Ig) variable chain (IgV) sequences provide a necessary first step towards developing therapeutic antibodies against pathogens. The virulence-associated Mtb antigens SodA (Superoxide Dismutase), KatG (Catalase), PhoS1/PstS1 (regulatory factor), and GroES (heat shock protein) are potential therapeutic targets but lacked IgV sequence characterization. Putative IgV sequences were identified from the mRNA of hybridomas targeting these antigens and isotype-switched into a common immunoglobulin fragment crystallizable region (Fc region) backbone, subclass IgG2aκ. Antibodies were validated by demonstrating recombinant Ig assembly and secretion, followed by the determination of antigen-binding specificity using ELISA and immunoblot assay.

Cloning and plant-based production of antibody MC10E7 for a lateral flow immunoassay to detect [4-arginine]microcystin in freshwater

Antibody MC10E7 is one of a small number of monoclonal antibodies that bind specifically to [Arg4]-microcystins, and it can be used to survey natural water sources and food samples for algal toxin contamination. However, the development of sensitive immunoassays in different test formats, particularly user-friendly tests for on-site analysis, requires a sensitive but also cost-effective antibody. The original version of MC10E7 was derived from a murine hybridoma, but we determined the sequence of the variable regions using the peptide mass-assisted cloning strategy and expressed a scFv (single-chain variable fragment) format of this antibody in yeast and a chimeric full-size version in leaves of Nicotiana tabacum and Nicotiana benthamiana to facilitate inexpensive and scalable production. The specific antigen-binding activity of the purified antibody was verified by surface plasmon resonance spectroscopy and ELISA, confirming the same binding specificity as its hybridoma-derived counterpart. The plant-derived antibody was used to design a lateral flow immunoassay (dipstick) for the sensitive detection of [Arg4]-microcystins at concentrations of 100-300 ng/L in freshwater samples collected at different sites. Plant-based production will likely reduce the cost of the antibody, currently the most expensive component of the dipstick immunoassay, and will allow the development of further antibody-based analytical devices and water purification adsorbents for the efficient removal of toxic contaminants.

Fab antibody fragment-functionalized liposomes for specific targeting of antigen-positive cells

Liposomes functionalized with monoclonal antibodies or their antigen-binding fragments have attracted much attention as specific drug delivery devices for treatment of various diseases including cancer. The conjugation of antibodies to liposomes is usually achieved by covalent coupling using cross-linkers in a reaction that might adversely affect the characteristics of the final product. Here we present an alternative strategy for liposome functionalization: we created a recombinant Fab antibody fragment genetically fused on its C-terminus to the hydrophobic peptide derived from pulmonary surfactant protein D, which became inserted into the liposomal bilayer during liposomal preparation and anchored the Fab onto the liposome surface. The Fab-conjugated liposomes specifically recognized antigen-positive cells and efficiently delivered their cargo, the Alexa Fluor 647 dye, into target cells in vitro and in vivo. In conclusion, our approach offers the potential for straightforward development of nanomedicines functionalized with an antibody of choice without the need of harmful cross-linkers.

Human IgG3 with extended half-life does not improve Fc-gamma receptor-mediated cancer antibody therapies in mice

Background

Current anti-cancer therapeutic antibodies that are used in the clinic are predominantly humanized or fully human immunoglobulin G1 (IgG1). These antibodies bind with high affinity to the target antigen and are efficient in activating the immune system via IgG Fc receptors and/or complement. In addition to IgG1, three more isotypes are present in humans, of which IgG3 has been found to be superior compared to human IgG1 in inducing antibody dependent cell cytotoxicity (ADCC), phagocytosis or activation of complement in some models. Nonetheless, no therapeutic human IgG3 mAbs have been developed due to the short in vivo half-life of most known IgG3 allotypes. In this manuscript, we compared the efficacy of V-gene matched IgG1 and IgG3 anti-tumour mAb (TA99) in mice, using natural variants of human IgG3 with short- or long half-life, differing only at position 435 with an arginine or histidine, respectively.

Results

In vitro human IgG1 and IgG3 did not show any differences in opsonisation ability of B16F10-gp75 mouse melanoma cells. IgG1, however, was superior in inducing phagocytosis of tumour cells by mouse macrophages. Similarly, in a mouse peritoneal metastasis model we did not detect an improved effect of IgG3 in preventing tumour outgrowth. Moreover, replacing the arginine at position 435 for a histidine in IgG3 to enhance half-life did not result in better suppression of tumour outgrowth compared to wild type IgG3 when injected prior to tumour cell injection.

Conclusion

In conclusion, human IgG3 does not have improved therapeutic efficacy compared to human IgG1 in a mouse tumour model.

Multi-level glyco-engineering techniques to generate IgG with defined Fc-glycans

Immunoglobulin G (IgG) mediates its immune functions through complement and cellular IgG-Fc receptors (FcγR). IgG contains an evolutionary conserved N-linked glycan at position Asn297 in the Fc-domain. This glycan consists of variable levels of fucose, galactose, sialic acid, and bisecting N-acetylglucosamine (bisection). Of these variations, the lack of fucose strongly enhances binding to the human FcγRIII, a finding which is currently used to improve the efficacy of therapeutic monoclonal antibodies. The influence of the other glycan traits is largely unknown, mostly due to lack of glyco-engineering tools. We describe general methods to produce recombinant proteins of any desired glycoform in eukaryotic cells. Decoy substrates were used to decrease the level of fucosylation or galactosylation, glycosyltransferases were transiently overexpressed to enhance bisection, galactosylation and sialylation and in vitro sialylation was applied for enhanced sialylation. Combination of these techniques enable to systematically explore the biological effect of these glycosylation traits for IgG and other glycoproteins.

Anti-tumor activity of human IgG1 anti-gp75 TA99 mAb against B16F10 melanoma in human FcgammaRI transgenic mice

Patients suffering from advanced melanoma have a very poor prognosis. Despite recent advances in the understanding of oncogenic mechanisms and therapeutic interventions, the median survival of patients with metastatic disease is less than 12 months. Immunotherapy of melanoma has been intensely investigated and holds great promises. Tyrosinase-related protein-1 or gp75 (TYRP-1/gp75) antigen is a melanosomal polypeptide. It is the most abundant glycoprotein synthesized by pigmented melanocytes and melanomas. It is specific for melanocytes and both primary and metastatic melanomas. In mice, administration of the mouse mAb anti-gp75 TA99 prevents outgrowth of B16F10 melanoma metastases. The activity of TA99 is dependent on the presence and activity of the IgG specific, Fc receptors. TA99 cross-reacts with human gp75, and is currently being used for diagnosis of patients. Here, we sequenced mIgG2a TA99 and found that the locus harboring the endogenous light chain of the fusion partner in the TA99 hybridoma cells is not inactivated, resulting in the production of a mixed pool of mAbs that mitigates binding to gp75. Since human IgG1 (hIgG1) is the most frequently used mAb format in clinical studies, we produced a recombinant hIgG1 TA99 molecule. Whereas it is known that hIgG1 can functionally interact with mouse Fc receptors, we found that hIgG1 TA99 did not exhibit in vivo activity against B16F10 melanoma in wild type C57BL/6 mice. However, results obtained in this study demonstrated anti-tumor activity of hIgG1 TA99 in FcγRIIB knockout mice and in human FcγRI transgenic mice. These results emphasize the need for testing hIgG mAb in mice with functional human FcγRs.

Intranasal administration of antibody-bound respiratory syncytial virus particles efficiently primes virus-specific immune responses in mice

Infants are protected from a severe respiratory syncytial virus (RSV) infection in the first months of life by maternal antibodies or by prophylactically administered neutralizing antibodies. Efforts are under way to produce RSV-specific antibodies with increased neutralizing capacity compared to the currently licensed palivizumab. While clearly beneficial during primary infections, preexisting antibodies might affect the onset of adaptive immune responses and the ability to resist subsequent RSV infections. Therefore, we addressed the question of how virus neutralizing antibodies influence the priming of subsequent adaptive immune responses. To test a possible role of the neonatal Fc receptor (FcRn) in this process, we compared the responses in C57BL/6 wild-type (WT) and FcRn(-/-) mice. We observed substantial virus-specific T-cell priming and B-cell responses in mice primed with RSV IgG immune complexes resulting in predominantly Th1-type CD4(+) T-cell and IgG2c antibody responses upon live-virus challenge. RSV-specific CD8(+) T cells were primed as well. Activation of these adaptive immune responses was independent of FcRn. Thus, neutralizing antibodies that localize to the airways and prevent infection-related routes of antigen processing can still facilitate antigen presentation of neutralized virus particles and initiate adaptive immune responses against RSV.

Enhanced ADCC activity of affinity maturated and Fc-engineered mini-antibodies directed against the AML stem cell antigen CD96

CD96, a cell surface antigen recently described to be preferentially expressed on acute myeloid leukemia (AML) leukemic stem cells (LSC) may represent an interesting target structure for the development of antibody-based therapeutic approaches. The v-regions from the CD96-specific hybridoma TH-111 were isolated and used to generate a CD96-specific single chain fragment of the variable regions (scFv). An affinity maturated variant resulting in 4-fold enhanced CD96-binding was generated by random mutagenesis and stringent selection using phage display. The affinity maturated scFv CD96-S32F was used to generate bivalent mini-antibodies by genetically fusing an IgG1 wild type Fc region or a variant with enhanced CD16a binding. Antibody dependent cell-mediated cytotoxicity (ADCC) experiments revealed that Fc engineering was essential to trigger significant effector cell-mediated lysis when the wild type scFv was used. The mini-antibody variant generated by fusing the affinity-maturated scFv with the optimized Fc variant demonstrated the highest ADCC activity (2.3-fold enhancement in efficacy). In conclusion, our data provide proof of concept that CD96 could serve as a target structure for effector cell-mediated lysis and demonstrate that both enhancing affinity for CD96 and for CD16a resulted in mini-antibodies with the highest cytolytic potential.

Isolation of alpaca anti-hapten heavy chain single domain antibodies for development of sensitive immunoassay

Some unique subclasses of Camelidae antibodies are devoid of the light chain, and the antigen binding site is comprised exclusively of the variable domain of the heavy chain (VHH). Although conventional antibodies dominate current assay development, recombinant VHHs have a high potential as alternative reagents for the next generation of immunoassay. We expressed VHHs from an immunized alpaca and developed a VHH-based immunoassay using 3-phenoxybenzoic acid (3-PBA), a major metabolite of pyrethroid insecticides as a model system. A phage VHH library was constructed, and seven VHH clones were selected by competitive binding with 3-PBA. The best immunoassay developed with one of these VHHs showed an IC(50) of 1.4 ng/mL (limit of detection (LOD) = 0.1 ng/mL). These parameters were further improved by using the phage borne VHH, IC(50) = 0.1 ng/mL and LOD = 0.01 ng/mL. Both assays showed a similar tolerance to methanol and dimethylsulfoxide up to 50% in assay buffer. The assay was highly specific to 3-PBA and its 4-hydroxylated derivative, 4-hydroxy 3-PBA, (150% cross reactivity) with negligible cross reactivity with other tested structural analogues, and the recovery from spiked urine sample ranged from 80 to 112%. In conclusion, a highly specific and sensitive VHH for 3-PBA was developed using sequences from immunized alpaca and phage display technology for antibody selection.

Identification of two aberrant transcripts derived from a hybridoma with amplification of functional immunoglobulin variable genes

Murine monoclonal antibodies (mAbs) are widely used but have limitations if administered in humans. The use of chimeric or humanized mAbs can reduce immunogenicity. The first step in producing such mAbs is to clone murine variable genes from a hybridoma, but it is possible to amplify both functional and aberrant variable genes, as they coexist in the hybridoma. During the development of a murine-human chimeric antibody, we have cloned from a hybridoma the functional heavy chain variable region (V(H)) and light chain variable region (V(L)) genes of a mAb that blocks the binding of anthrax lethal factor to protective antigen. In this study, we report the detection of two aberrant transcripts from a hybridoma produced using myeloma cell line OUR-1, the development of a method to distinguish between the functional and abundant aberrant V(L) transcripts, and the origins of these aberrant genes. The aberrant V(L) gene is derived from OUR-1 cells, while the aberrant V(H) gene might derive from antibody repertoires in B cells or from gene rearrangement in the hybridoma cells. The aberrant V(H) and V(L) genes in this study may facilitate discrimination between the functional and aberrant variable genes from hybridoma cells.

Peptide mass-assisted antibody cloning strategy for accurate characterization of potential therapeutic monoclonal antibodies against neurodegenerative diseases

The development of therapeutic recombinant antibodies involves accurate characterization of immunoglobulin variable light (VL) and heavy (VH) chains. However, it has been reported that the use of subgroup or isotype-specific primers for the amplification of monoclonal antibody (mAb) variable domains introduces heterogeneities within the variable domains, or amplifies aberrant productive Ig domains. To address these issues, we have combined the rapid amplification of cDNA ends PCR (RACE-PCR) for the full-length VL and VH amplification, with peptide mass fingerprinting of the corresponding Ig chain. Using this strategy, we amplified full-length cDNA chains of SAF34 and SAF32, two potential therapeutic mAbs against neurodegenerative diseases directed to the prion protein (PrP). We report an unambiguous correlation between hybridoma cDNA sequences and protein fingerprints of the variable domains of each mAb, indicating the discrimination between mutated, pseudo-genes and functional Ig genes. As a proof of principle for this dual strategy of full-length PCR amplification of variable domains and their characterization by MALDI-TOF, we show that the corresponding scFvs recognize the native PrP and retain full capacity to bind to human PrP, as does the parental mAb. This finding addresses the need for reliable light and heavy chain characterization, a key factor for humanization of mouse antibodies and for its use in passive immunotherapy applications.

Epstein-Barr viruses that express a CD21 antibody provide evidence that gp350's functions extend beyond B-cell surface binding

The gp350 glycoprotein encoded by BLLF1 is crucial for efficient Epstein-Barr virus (EBV) infection of resting B cells. Gp350 binds to CD21, but whether this interaction sums up its functions remains unknown. We generated gp350-null EBVs that display CD19-, CD21-, or CD22-specific antibodies at their surface (designated as DeltaBLLF1-Ab). Gp350-complemented (DeltaBLLF1-C) and DeltaBLLF1-Ab were found to bind equally well to B cells. Surprisingly, DeltaBLLF1 binding was reduced only 1.7-fold relative to its complemented counterparts. Furthermore, B cells exposed to DeltaBLLF1-Ab or DeltaBLLF1 viruses presented structural antigens with comparable efficiency and achieved 25 to 80% of the T-cell activation elicited by DeltaBLLF1-C. These findings show that the gp350-CD21 interaction pair plays only a modest role during virus transfer to the endosomal compartment. However, primary B cells or Raji B cells infected with DeltaBLLF1-C viruses displayed a 35- to 70-fold higher infection rates than those exposed to DeltaBLLF1, DeltaBLLF1-CD22Ab, or DeltaBLLF1-CD19Ab viruses. Complementation of the gp350 knockout phenotype with CD21Ab substantially enhanced infection rates relative to DeltaBLLF1 but remained sevenfold (Raji B-cell line) to sixfold (primary B cells) less efficient than with gp350. We therefore infer that gp350 mainly exerts its functions after the internalization step, presumably during release of the viral capsid from the endosomal compartment, and that CD21-dependent but also CD21-independent molecular mechanisms are involved in this process. The latter appear to be characteristic of B-cell infection since transfection of CD21 in 293 cells improved the infection rates with both DeltaBLLF1-CD21Ab and DeltaBLLF1-C to a similar extent.

Clonal relationships between thyroid-stimulating hormone receptor-stimulating antibodies illustrate the effect of hypermutation on antibody function

Graves' disease is characterized by production of agonist antibodies to the thyroid-stimulating hormone receptor (TSHR), but knowledge of the genetic and somatic events leading to their aberrant production is limited. We describe the genetic analysis of two monoclonal antibodies (mAbs) with thyroid-stimulating activity (TSAb) obtained from a single mouse with experimental Graves' disease. The mAbs were class switched, but used the same rearrangement of immunoglobulin heavy chain, variable region (IGHV) and immunoglobulin light chain, variable region (IGLV) germline genes, implying a clonal relationship and derivation from a single precursor B-cell clone. The IGHV-region genes of the two mAbs underwent high degrees of somatic hypermutation by sharing numerous mutations before diverging, while the IGLV genes evolved separately. Interestingly, the mutations were present in both the complementarity-determining regions (CDRs) and the framework regions. The cloned IGHV and IGLV genes were confirmed to have TSAb properties in experiments in which they were expressed as recombinant Fabs (rFabs). In other experiments, we swapped the IGLV genes with IGHV genes by constructing chimeric rFabs and showed that the chimeras retained TSAb activities, confirming the close functional relatedness of the V-region genes. Importantly, the IGLV genes in chimeric rFabs had a dominant stimulatory effect at low concentrations, while the IGHV genes had a dominant effect at higher concentrations. Our findings demonstrate that, in experimentally immunized mice, multiple pathogenic antibodies to TSHR can arise from a single clone by a series of somatic mutations in the V-region genes and may give an insight into how such antibodies develop spontaneously in autoimmune Graves' disease.

Degenerated primer design to amplify the heavy chain variable region from immunoglobulin cDNA

BACKGROUND

The amplification of variable regions of immunoglobulins has become a major challenge in the cloning of antibody genes, whether from hybridoma cell lines or splenic B cells. Using conventional protocols, the heavy-chain variable region genes often are not amplified successfully from the hybridoma cell lines.

RESULTS

A novel method was developed to design the degenerated primer of immunoglobulin cDNA and to amplify cDNA ends rapidly. Polymerase chain reaction protocols were performed to recognize the VH gene from the hybridoma cell line. The most highly conserved region in the middle of the VH regions of the Ig cDNA was identified, and a degenerated 5'primer was designed, using our algorithms. The VH gene was amplified by both the 3'RACE and 5'RACE. The VH sequence of CSA cells was 399 bp.

CONCLUSION

The new protocol rescued the amplifications of the VH gene that had failed under conventional protocols. In addition, there was a notable increase in amplification specificity. Moreover, the algorithm improved the primer design efficiency and was shown to be useful both for building VH and VL gene libraries and for the cloning of unknown genes in gene families.

Crystal Structure of an Anti-meningococcal Subtype P1.4 PorA Antibody Provides Basis for Peptide–Vaccine Design

In various western countries, subtype P1.4 of Neisseria meningitidis serogroup B causes the greatest incidence of meningococcal disease. To investigate the molecular recognition of this subtype, we crystallised a peptide (P1HVVVNNKVATH(P11)), corresponding to the subtype P1.4 epitope sequence of outer membrane protein PorA, in complex with a Fab fragment of the bactericidal antibody MN20B9.34 directed against this epitope. Structure determination at 1.95 A resolution revealed a unique complex of one P1.4 antigen peptide bound to two identical Fab fragments. One Fab recognises the putative epitope residues in a 2:2 type I beta-turn at residues P5NNKV(P8), whereas the other Fab binds the C-terminal residues of the peptide that we consider a crystallisation artefact. Interestingly, recognition of the P1.4 epitope peptide is mediated almost exclusively through the complementarity-determining regions of the heavy chain. We exploited the observed turn conformation for designing conformationally restricted cyclic peptides for use as a peptide vaccine. The conformational stability of the two peptide designs was assessed by molecular dynamics simulations. Unlike the linear peptide, both cyclic peptides, conjugated to tetanus toxoid as a carrier protein, elicited antibody responses in mice that recognised meningococci of subtype P1.7-2,4. Serum bactericidal assays showed that some, but not all, of the sera induced with the cyclic peptide conjugates could activate the complement system with titres that were very high compared to the titres induced by complete PorA protein in its native conformation administered in outer membrane vesicles.

A general method allowing the design of oligonucleotide primers to amplify the variable regions from immunoglobulin cDNA

The amplification of variable regions of immunoglobulins by reverse transcription polymerase chain reaction (RT-PCR) has become an invaluable technique either for the cloning of monoclonal antibodies (mAbs), or for the building of single-chain fragment variable (ScFv) libraries. Numerous applications have been described either for studying the antigen-antibody interactions or for medical purposes, with the recent development of recombinant antibodies for therapeutic use. Several publications by different groups have reported primer sequences to perform such amplification, but the strategy used to design these primers, and particularly the way of performing the necessary alignments, generally appear poorly detailed. In the present work, we propose a rational method of designing primers in order to amplify the variable region of heavy chain (VH) and variable region of light chain (VL) domains for framework 1 (FR1) of immunoglobulins. The described sets of primers have been designed to hybridize with the entire VH and VL mouse repertory without modification of amino acids since amino acids of framework 1 play a role in the folding, and thus in the functionality, of recombinant antibody. These primers have been applied to the cloning of monoclonal antibodies previously produced in the laboratory. This approach can be extended to other species or members of the immunoglobulin superfamily.

Central role of complement in passive protection by human IgG1 and IgG2 anti-pneumococcal antibodies in mice

Streptococcus pneumoniae is an important cause of morbitity and mortality worldwide. Capsule-specific IgG1 and IgG2 Abs are induced upon vaccination with polysaccharide-based vaccines that mediate host protection. We compared the protective capacity of human recombinant serogroup 6-specific IgG1 and IgG2 Abs in mice deficient for either leukocyte FcR or complement factors. Human IgG1 was found to interact with mouse leukocyte FcR in vitro, whereas human IgG2 did not. Both subclasses induced complement activation, resulting in C3c deposition on pneumococcal surfaces. Passive immunization of C57BL/6 mice with either subclass before intranasal challenge with serotype 6A induced similar degrees of protection. FcgammaRI- and III-deficient mice, as well as the combined FcgammaRI, II, and III knockout mice, were protected by passive immunization, indicating FcR not to be essential for protection. C1q or C2/factor B knockout mice, however, were not protected by passive immunization. Passively immunized C2/factor B(-/-) mice displayed higher bacteremic load than C1q(-/-) mice, supporting an important protective role of the alternative complement pathway. Spleens from wild-type and C1q(-/-) mice showed hyperemia and thrombotic vessel occlusion, as a result of septicemic shock. Notably, thrombus formation was absent in spleens of C2/factor B(-/-) mice, suggesting that the alternative complement pathway contributes to shock-induced intravascular coagulation. These studies demonstrate complement to play a central role in Ab-mediated protection against pneumococcal infection in vivo, as well as in bacteremia-associated thrombotic complications.

Chimeric IgA antibodies against HLA class II effectively trigger lymphoma cell killing

Antibodies against human leukocyte antigen (HLA) class II, such as 1D10 or Lym-1, are currently being evaluated for the treatment of B-cell lymphomas. Previous studies have demonstrated that, in addition to IgG Fc receptors, the human myeloid IgA receptor (Fc(alpha)RI, CD89) also effectively triggered tumor cell killing. Therefore, we used the variable light and heavy chain sequences from another murine anti-HLA class II hybridoma, F3.3, to generate a panel of chimeric human/mouse antibodies, including human immunoglobulin A1 (IgA1), IgA2, IgG1, IgG2, IgG3, and IgG4. Antibody production was accomplished by stable transfection of baby hamster kidney cells, and binding activity and specificity were confirmed by enzyme-linked immunosorbent assay (ELISA) and Western blotting. All constructs demonstrated similar binding to HLA class II. Functional studies revealed that chimeric IgG1, IgA1, and IgA2 triggered similar levels of tumor cell lysis. Analyses of effector populations, however, demonstrated that killing by chimeric IgG1 constructs was triggered mainly by human mononuclear cells and complement, while IgA1 and IgA2 mediated effective lysis by polymorphonuclear neutrophils. Importantly, IgG1 and both IgA isotypes were equally effective at killing freshly isolated human chronic lymphocytic leukemia cells. Chimeric IgA antibodies against HLA class II may constitute attractive reagents for lymphoma therapy.

Activity of human IgG and IgA subclasses in immune defense against Neisseria meningitidis serogroup B

Both IgG and IgA Abs have been implicated in host defense against bacterial infections, although their relative contributions remain unclear. We generated a unique panel of human chimeric Abs of all human IgG and IgA subclasses with identical V genes against porin A, a major subcapsular protein Ag of Neisseria meningitidis and a vaccine candidate. Chimeric Abs were produced in baby hamster kidney cells, and IgA-producing clones were cotransfected with human J chain and/or human secretory component. Although IgG (isotypes IgG1-3) mediated efficient complement-dependent lysis, IgA was unable to. However, IgA proved equally active to IgG in stimulating polymorphonuclear leukocyte respiratory burst. Remarkably, although porin-specific monomeric, dimeric, and polymeric IgA triggered efficient phagocytosis, secretory IgA did not. These studies reveal unique and nonoverlapping roles for IgG and IgA Abs in defense against meningococcal infections.