A Novel Sensitive Cell-Based Immunoenzymatic Assay for Palytoxin Quantitation in Mussels

The marine algal toxin palytoxin (PLTX) and its analogues are some of the most toxic marine compounds. Their accumulation in edible marine organisms and entrance into the food chain represent their main concerns for human health. Indeed, several fatal human poisonings attributed to these compounds have been recorded in tropical and subtropical areas. Due to the increasing occurrence of PLTX in temperate areas such as the Mediterranean Sea, the European Food Safety Authority (EFSA) has suggested a maximum limit of 30 µg PLTX/kg in shellfish meat, and has recommended the development of rapid, specific, and sensitive methods for detection and quantitation of PLTX in seafood. Thus, a novel, sensitive cell-based ELISA was developed and characterized for PLTX quantitation in mussels. The estimated limits of detection (LOD) and quantitation (LOQ) were 1.2 × 10⁻¹¹ M (32.2 pg/mL) and 2.8 × 10⁻¹¹ M (75.0 pg/mL), respectively, with good accuracy (bias = 2.5%) and repeatability (15% and 9% interday and intraday relative standard deviation of repeatability (RSDr), respectively). Minimal interference of 80% aqueous methanol extract allows PLTX quantitation in mussels at concentrations lower than the maximum limit suggested by EFSA, with an LOQ of 9.1 µg PLTX equivalent/kg mussel meat. Given its high sensitivity and specificity, the cell-based ELISA should be considered a suitable method for PLTX quantitation.

Characterization of palytoxin binding to HaCaT cells using a monoclonal anti-palytoxin antibody

Palytoxin (PLTX) is the reference compound for a group of potent marine biotoxins, for which the molecular target is Na+/K+-ATPase. Indeed, ouabain (OUA), a potent blocker of the pump, is used to inhibit some PLTX effects in vitro. However, in an effort to explain incomplete inhibition of PLTX cytotoxicity, some studies suggest the possibility of two different binding sites on Na+/K+-ATPase. Hence, this study was performed to characterize PLTX binding to intact HaCaT keratinocytes and to investigate the ability of OUA to compete for this binding. PLTX binding to HaCaT cells was demonstrated by immunocytochemical analysis after 10 min exposure. An anti-PLTX monoclonal antibody-based ELISA showed that the binding was saturable and reversible, with a K(d) of 3 × 10-10 M. However, kinetic experiments revealed that PLTX binding dissociation was incomplete, suggesting an additional, OUA-insensitive, PLTX binding site. Competitive experiments suggested that OUA acts as a negative allosteric modulator against high PLTX concentrations (0.3-1.0 × 10-7 M) and possibly as a non-competitive antagonist against low PLTX concentrations (0.1-3.0 × 10-9 M). Antagonism was supported by PLTX cytotoxicity inhibition at OUA concentrations that displaced PLTX binding (1 × 10-5 M). However, this inhibition was incomplete, supporting the existence of both OUA-sensitive and -insensitive PLTX binding sites.

Sandwich ELISA assay for the quantitation of palytoxin and its analogs in natural samples

Palytoxins are potent marine biotoxins that have recently become endemic to the Mediterranean Sea, and are becoming more frequently associated with seafood. Due to their high toxicity, suitable methods to quantify palytoxins are needed. Thus, we developed an indirect sandwich ELISA for palytoxin and 42-hydroxy-palytoxin. An intralaboratory study demonstrated sensitivity (limit of detection, LOD = 1.1 ng/mL; limit of quantitation, LOQ = 2.2 ng/mL), accuracy (bias of 2.1%), repeatability (RSDr = 6% and 9% for intra- and interassay variability, respectively) and specificity: other common marine toxins (okadaic acid, domoic acid, saxitoxin, brevetoxin-3, and yessotoxin) do not cross-react in this assay. It performed well in three different matrices: observed LOQs were 11.0, 9.6, and 2.4 ng/mL for mussel extracts, algal net samples and seawater, respectively, with good accuracy and precision. The LOQ in seafood is 11 μg palytoxin/kg mussel meat, lower than that of the most common detection technique, LC-MS/MS.

Harmful dinoflagellate Ostreopsis cf. ovata Fukuyo: detection of ovatoxins in field samples and cell immunolocalization using antipalytoxin antibodies

Ostreopsis cf. ovata, a benthic dinoflagellate often blooming along the Mediterranean coasts, has been associated with toxic events ranging from dyspnea to mild dermatitis. In late September 2009, an Ostreopsis cf. ovata bloom occurred in the Gulf of Trieste (Northern Adriatic Sea; Italy), causing pruritus and mild dermatitis in beachgoers. An integrated study was initiated to characterize Ostreopsis cells by light and confocal microscopy, PCR techniques, immunocytochemistry, and high resolution liquid chromatography-mass spectrometry (HR LC-MS). The presence of Ostreopsis cf. ovata of the Atlantic/Mediterranean clade was unambiguously established by morphological and genetic analyses in field samples. Several palytoxin-like compounds (ovatoxin-a,-b,-c,-d,-e) were identified by HR LC-MS, ovatoxin-a being the most abundant (45-64 pg/cell). Surprisingly, no palytoxin was detected. For the first time, monoclonal and polyclonal antipalytoxin antibodies revealed the intracellular cytoplasmic localization of ovatoxins, suggesting their cross-reactivity with these antibodies. Since harmful dinoflagellates do not always produce toxins, the immunocytochemical localization of ovatoxins, although qualitative, can provide an early warning for toxic Ostreopsis cells before their massive diffusion and/or concentration in seafood.

Comparative analysis of purified Pacific and Caribbean ciguatoxin congeners and related marine toxins using a modified ELISA technique

The monoclonal antibody to ciguatoxin (CTX) produced from a hybridoma cell line was assayed for the detection of four congeners of CTX: Pacific ciguatoxin-1 (P-CTX-1), Pacific ciguatoxin-2 (P-CTX-2), Pacific ciguatoxin-3 (P-CTX-3), and Caribbean ciguatoxin-1 (C-CTX-1) and related marine toxins, including domoic acid, palytoxin, and okadaic acid, using a modified enzyme-linked immunosorbent assay (ELISA). Lower detection limits were assessed and linearity was statistically established (P<0.05) for P-CTX-1, P-CTX-2, and P-CTX-3 and C-CTX-1 at concentrations ranging from 0 to 5.00 ng, while the other marine toxins showed statistically insignificant cross-reactivities at similar concentrations. Thus, the monoclonal antibody to CTX is able to specifically detect various CTX congeners at levels comparable to those naturally occurring in ciguatoxic fish.

Water-soluble covalent conjugates of bovine serum albumin with anionic poly(N-isopropyl-acrylamide) and their immunogenicity

We have conjugated bovine serum albumin (BSA) to poly(N-isopropylacrylamide-co-acrylic acid) (poly(NIPAAm-AA)) by using water-soluble carbodiimide, and the effects of the bulk mass ratio of protein to polymer (r) on the formation of polymer-protein conjugates have been studied. HPLC, electrophoresis, viscosimetry and fluorescence spectroscopy suggest that the mode of covalent binding of BSA to poly(NIPAAm-AA) depends upon the weight concentration ratio (r) of BSA to poly(NIPAAm-AA). At r < or = 1, free poly(NIPAAm-AA) molecules coexist with conjugate, and when r reaches 1 the amount of free polymer is too small to be observed. It is shown that depending on the ratio r, two types of conjugate particles were formed: at r < 1, the protein molecules in the structure of conjugate particles are densely covered as a shell by polymer chain and practically "fenced off" from water environment; at r > 1 the conjugate-forming particles possess more friable structures in which protein molecules are practically exposed to the solvent. The complex formation involving electrostatic interactions between BSA and carbodiimide activated polymer are proposed as the driving force for the covalent binding of BSA to polymer macromolecules. The coil-globule transition of macromolecules in low and thermally induced precipitation in more concentrated solutions of bioconjugates was observed. The immunogenic properties of covalent conjugates of CP-BSA were investigated and the temperature-modulated solubility-immunogenicity alterations was analyzed. A single immunization of mice with conjugates at the thermally precipitating concentration without an adjuvant evoked increased specific immune response to BSA, which practically did not depend on the initial conjugation ratio of components. Such a modulated system is attractive for application as a novel immunogenic system in vaccine technology.

Synthesis of allyl 6-O-(3-deoxy-alpha- and -beta-D-manno-oct-2- ulopyranosylonic acid)-(1----6)-2-deoxy-2-[(3R)-3-hydroxytetradecanamido]- beta-D-glucopyranoside 4-phosphate and of the copolymer of the alpha anomer with acrylamide

The title disaccharides were synthesized by mercuric cyanide-catalyzed condensation of methyl (4,5,7,8-tetra-O-acetyl-3-deoxy-alpha-D-manno-oct-2-ulopyranosyl bromide)onate and allyl 2-[(3R)-3-acetoxy-tetradecanamido]-3-O-benzyl-2-deoxy-beta-D- glucopyranoside, followed by phosphorylation of the alcoholic function of the amino sugar. The phosphorylated anomers were separated by chromatography and deprotected by conventional methods. Polymeric material was obtained by copolymerisation, catalyzed by peroxosulphate and N,N,N',N'-tetramethylethylenediamine, of the alpha anomer with acrylamide; it contained ca. one disaccharide unit for 18 acrylamide residues.

Synthesis of polyamide supports for use in peptide synthesis and as peptide-resin conjugates for antibody production

We have synthesized beaded, hydrophilic cross-linked, aminoalkyl polydimethylacrylamide supports upon which peptides have been assembled using standard Boc or Fmoc chemistry in automated equipment. The resins were prepared by the free radical-initiated co-polymerization of N,N-dimethylacrylamide, N,N'-bisacrylyl-1,3-diaminopropane, and a functional monomer which were contained in a reverse-phase, detergent-emulsified suspension. The functional monomers used were N-(2-(methylsulfonyl)ethyloxycarbonyl)-allyl-amine (MSC-allylamine), N-acrylyl-1,6-diaminohexane hydrochloride or N-methacrylyl-1,3-diamino-propane hydrochloride. The MSC protecting group was removed by treatment of the resin with methanolic base during workup. After coupling of N-alpha-t-butyloxycarbonyl-alanine (Boc-alanine), amino acid analyses gave resin loading capacities between 0.15 mmol/g and 1.4 mmol/g, depending on the concentration and composition of the functional monomer. The resulting polymers were highly swollen by polar solvents including aqueous buffers. Peptides were synthesized on these supports after attaching the first amino acid directly or through a cleavable ester linker. When the carboxyl-terminal amino acid was coupled as the 4-oxymethylbenzoic acid derivative, the peptide could be deprotected and remain attached to the hydrophilic polymer since the peptide-benzyl ester bond was stable to HF deprotection at 0 degrees in the presence of 10% anisole and 1% ethanedithiol. The resulting peptidyl-resin could be swollen in aqueous buffers and injected into animals for the production of antibodies.

[Immunochemical characteristics of artificial antigens with Salmonella O-determinants 4 and 9]

Two new synthetic high-molecular protein-free O-antigens have been obtained. These O-antigens are copolymers of synthetic disaccharides with acrylamide. The copolymers of alpha-allylglycosides, abequosyl-(alpha 1----3]-mannopyranose and tyvelosyl-(alpha 1----3)-mannopyranose, with acrylamide have been shown to possess, respectively, the specificity of immunodominant O-factors 4 and 9 characteristic of Salmonella serogroups B and D1. The synthetic O-antigens have proved to possess higher capacity for binding antibodies in the precipitation test and the hemagglutination inhibition test than the corresponding lipopolysaccharides.

Effect of the chemical structure of N-(2-hydroxypropyl)methacrylamide copolymers on their ability to induce antibody formation in inbred strains of mice

The homopolymer of N-(2-hydroxypropyl)methacrylamide (HPMA) and copolymers of HPMA differing in oligopeptide side chains (-Gly-Gly-OH; -Acap-Phe-OH; -Acap-Leu-HMDA and -Gly-Phe-Tyr-OH) or in their content (1%, 3.5% and 8.4% mole of -Gly-Gly-OH side chains) were investigated with respect to their ability to induce antibody formation and mitogenic reaction in inbred strains of mice. The dependence on the antigen dose, on composition of the side chain and on the genetic background of the immunized organism was defined. It was demonstrated that the specificity of the antibody formed is predominantly directed against oligopeptide side chains, though some part of the antibody is also produced against hydroxypropyl chains. Neither the homopolymer nor the copolymers behave in the tissue culture as mitogens.

Immunogenicity of N-(2-hydroxypropyl)-methacrylamide copolymers--potential hapten or drug carriers

After repeated i.p. immunizations of mice with 10 micrograms of homopolymer poly (HPMA) no antibodies were detected by the ELISA test. Immunization with copolymer P-Acap-Leu-HMDA leads to a weak antibody response, while immunization with a copolymer with some side chains modified with ARS or FITC groups (P-Acap-Leu-HMDA-ARS or P-Acap-Leu-HMDA-FITC) leads to a significant antibody response detectable by PFC, ELISA and haemagglutination tests. Most of these antibodies are aimed against the modifying haptenic group, a smaller amount against side oligopeptide sequences of the carrier. Intensity of the antibody response depends on: 1) the antigen dose--the optimal dose was 10 micrograms: both the higher (100 micrograms) and the lower doses (1 and 0.1 micrograms) induced considerably lower antibody responses; 2) molar mass of the immunizing fractions--fractions of high molar mass induced up to five times higher responses than those of a low molar mass; 3) the bound haptenic group--the ARS-copolymers induced ten times lower response than the FITC-copolymers. We detected no difference between capacities of the H-2a, H-2b and H-2d haplotypes to react with anti-ARS antibodies after immunization with P-Acap-Leu-HMDA-ARS.

[Immunological properties of an artificial antigen possessing the specificity of Salmonella O-factor 3]

For the first time the method of obtaining synthetic protein-free antigen with the specificity of Salmonella O-factor by the radical copolymerization of the synthetic trisaccharide 3-0 [4-0-(beta-D-mannopyranosyl)-alpha-L-rhamnopyranosyl]-beta-allyl-D- galactopyranoside with acrylamide was developed. The synthetic antigen thus obtained possessed the narrow specificity of serogroup E Salmonella O-factor 3. The serological activity of the antigen, studied in the precipitation and passive hemagglutination tests, was considerably higher than that of lipopolysaccharides isolated from S. anatum and S. newington. This synthetic antigen proved to be nontoxic and possessed immunogenic properties, inducing the formation of antibodies to Salmonella O-factor 3 in immunized animals.

[Preventive activity of an artificial antigen possessing the serological specificity of Salmonella O-factor 3]

3-0 [4-0 (beta-D-mannopyranosyl)-alpha-L-rhamnopyranosyl]-beta-allyl-D- galactopyranoside with acrylamide, a new synthetic antigen possessing the narrow specificity of Salmonella O-factor 3, was found to protect mice, when introduced into the animals by multiple intraperitoneal injections, from the action of the live culture of Salmonella anatum and the toxic doses of its endotoxin.

Regulation of immune response in allogeneic mixed spleen cell cultures. I. Influence of I-region on the generation of suppressor cells

The immune responses of allogeneic mixed spleen cell cultures (MLC) to the T-dependent antigen, SRBC, and to the T-independent antigen, DNP-PAA, were investigated. The immune response to DNP-PAA in MLC with certain strain combinations was always suppressed as compared with the expected PFC response calculated from the PFC responses of the individual strains. This suppression was eliminated by treating the spleen cells with RAMB antiserum plus complement before the incubation of the MLC with DNP-PAA. It can be concluded that the suppression in the PFC response to the T-independent antigen DNP-PAA in MLC is due to the generation of suppressor T-cells. The PFC response to the T-dependent antigen, SRBC, in MLC showed either suppression, no change, or rarely augmenation, suggesting that the allogeneic mixed spleen cell cultures can generate both suppressor and helper T cells and that the balance between helper and suppressor activity regulates the PFC response to a T-dependent antigen. Suppressor activity was also generated in a one-way MLC, but the degree of suppression depended upon which of the two strains was responding. Similar amounts of thymidine were incorporated in the one-way MLR irrespective of which strains was responding. Thus, the extent of proliferation in one-way MLR is not related to the degree of suppressor activity generated. The results further indicate that a difference between two strains in the I-C, S, and G regions of the major histocompatibility complex is required to generate suppressor activitiy that can depress the response to a T-independent antigen, MLC between strains differing in K, I-A, I-B, I-J, I-E, and D regions generate little or no suppressor activity in this system.

The murine B cell response to TNP-polyacrylamide beads: the relationship between the epitope density of the antigen and the requirements for T cell help and surface IgD

Spleen cells were cultured with high or low epitope TNP-polyacrylamide beads (TNP-PAB) in order to investigate the effect of epitope density on the requirements for T cell help and surface IgD on responding B cells. The response to low epitope density TNP-PAB was abolished by treatment with anti-Thy-1.2 and complement, whereas approximately 50% of the response to high epitope density TNP-PAB was retained after similar treatment. Thus, an increase in epitope density resulted in a decreased requirement for T cell help. An increase in epitope density was also associated with a decreased requirement for interaction of antigen and surface IgD as determined by "blocking" studies with anti-delta; further, the majority of the T-independent portion of this response was not blocked by anti-delta antibodies. This finding indicate that the T-independent portion of the anti-TNP response does not require interaction of antigen with surface IgD on B cells. These results are discussed in terms of differential cross-linking of IgM and IgD receptors on B cells by multivalent antigens.

Regulation of the primary in vitro antibody response in human peripheral blood lymphocytes: different effects of mitogen-induced and spontaneous T suppressor cells

The specific response of human peripheral blood lymphocyte cultures to TNP-polyacrylamide was suppressed by the addition of concanavalin A (Con A). A dose of Con A (0.5 microgram/ml) could be selected, which induced a reproducible but incomplete suppression independent of the magnitude of the anti-TNP response. Con A-stimulated cells could transfer the suppression to autologous or allogeneic responding cells. The suppressor activity was present in the E-rosette forming cell fraction and was abolished by mitomycin C treatment prior to incubation. With a particular batch of foetal bovine serum, spontaneous suppressor cells were produced which suppressed the response of autologous and allogeneic lymphocytes. In allogenic mixtures, the otherwise enhancing allogeneic effect was replaced by a marked suppression from spontaneous suppressor cells. This suppression was higher than that exerted on autologous lymphocytes, suggesting that an unexpected negative allogenic effect had taken place. Spontaneous suppressors were ineffective when added on day 2 of a culture responding to TNP-polyacrylamide, whereas Con A induced suppressors were fully effective.

Studies on immunosuppression by cobra venom factor. II. On responses to DNP-Ficoll and DNP-Polyacrylamide

We have reassessed the effects of CVF administration on the humoral responses to two T-independent immunogens: DNP-Ficoll and DNP-Polyacrylamide. With high immunogen doses, little or no evidence of suppression was found. However, when the immunizing dose was reduced, suppression of both IgG and IgM responses became apparent. As indicated in a previous report, the immunosuppressive effect of CVF on T-dependent responses may result not only from C depletion but also from the generation of C cleavage products that may impair the auxiliary contribution of macrophages to the generation of these humoral responses. A similar mechanism may be applicable to the suppression of antibody production to DNP-Ficoll and DNP-PAA in view of recent reports showing a macrophage requirement for the response to these immunogens.

Cellular basis for a hapten-specific state of tolerance induced in vitro

A hapten-specific unresponsive state was induced in vitro by the incubation of normal murine spleen cells with highly conjugated dinitrophenylated bovine gamma-globulin (DNP-BGG) or a dinitrophenylated copolymer of D-glutamic acid and D-lysine (DNP-D-GL) for 24 hr. After this incubation period spleen cells were washed and cultured for 4 days with the thymic-independent antigen dinitrophenylated polyacrylamide beads (DNP-PAA) or the thymic-dependent antigen trinitrophenylated burro the erythrocytes (TNP-BRBC). Preincubation with either DNP-BGG or DNP-D-GL led to a specific depression of the in vitro anti-hapten plaque-forming cell response. The degree of depression was dependent upon the concentration of the tolerogen and the duration of preincubation. The response to DNP-PAA or TNP-PAA beads was depressed to a greater degree than was the response to TNP-BRBC. The cellular basis of the immunologic unresponsiveness induced by DNP-BGG was attributable to an inhibition of B cell function whereas the unresponsive state induced with DNP-D-GL was due to both a specific inhibition of B cell function and the activation of antigen-specific suppressor T cells.

The immunogenicity of dinitrophenylated polyacrylamide and the adjuvant effect of polyacrylamide

Dinitrophenyl-substituted polyacrylamide (PAAm-DNP) is weakly immunogenic in guinea pigs and rats. Only 2-mercaptoethanol-sensitive IgM antibodies are elicited, and no immunological memory in guinea pigs is appreciated. A 15000-fold carrier excess, given 6 days before an immunogenic dose of PAAm-DNP, does not impair the anti-DNP response. Different doses of PAAm failed to increase the level of natural anti-DNP or anti-sulfanilic acid antibodies, either in guinea pigs or rats. Nevertheless, when given together with sheep red blood cells. PAAm increases the anti-sheep red blood cell humoral immune response in rats. It is suggested from the above data that the PAAm-DNP immunogenicity is not accounted for by the 'two-signal' model of B-cell activation (via 'associative antibodies'), nor by a model that ascribes B-cell activation to the polyclonal B-cell activating properties of the carrier. However, the possibility that the adjuvant effect showed by PAAm plays a role in allowing the conjugate PAAm-DNP to be immunogenic, remains an open question, and must be further investigated.

Enhancing effect of low dose cyclophosphamide treatment on the in vitro antibody response

We have studied the effect of cyclophosphamide (CY) administration on the subsequent in vitro antibody response in the mouse. Treatment with a low dose (20 mg/kg) of CY four days before culture results in an increased IgM response to the T-independent antigen trinitrophenylated polyacrylamide (TNP-PAA), without affecting the background response of unstimulated cultures. This suggests that CY treatment eliminates a short-lived suppressor cell, involved in the regulation of the in vitro B cell response. In contrast, the same regimen decreases the ability of nude mouse spleen cells to respond to TNP-PAA, showing that the target of CY-enhancing effect is a mature T cell. The increased response observed in conventional mice should be the result of a balance between the direct suppressive effect of CY on B cells and the elimination of a suppresor T cell, the latter phenomenon being of predominant significance in our conditions. The target of CY-enhancing effect is nonadherent to plastic, but adherent to Sephadex G-10 columns.

Primary in vitro antibody response from human peripheral blood lymphocytes

A method for the induction of a primary in vitro antibody response from human peripheral blood lymphocytes is presented. Upon cultivation with trinitrophenyl conjugated polyacrylamide beads (TNP-PAA), an anti-TNP response can be obtained as indicated by the appearance of direct plaque-forming cells from day 5 of culture, with a reproducible peak on day 8. These plaques correspond to cells actively producing antibody of the IgM type, as shown by their inhibition by cycloheximide and by anti-human IgM serum, but not by anti-human Fc gamma serum. Their specificity for the TNP hapten can be demonstrated by the effector cell blockade phenomenon, with highly substituted TNP-human IgG. Although the anti-TNP response induced by TNP-PAA in mouse spleen cell cultures appears T independent the same response in human PBL may involve in addition the participation of T cells, since E-RFC depletion before culture led to a markedly decreased number of plaque-forming cells. A significant response could be obtained from the PBL of all of the 30 normal individuals tested. Importantly, the response was reproducible in its magnitude in the six individuals tested in at least three different experiments. Thus, the in vitro stimulation of human PBL by TNP-PAA can be proposed as a reliable test for the study of human B cell function in a specific primary antibody response.

Molecular determinants of immunogenicity: the immunon model of immune response

The immunological response in vivo to a series of size-fractionated linear polymers of acrylamide substituted with hapten has been measured in mice. A sharp threshold was observed in immunogenic response elicited by various polymer preparations. All polymers with less than 12 to 16 appropriately spaced hapten groups per molecule were nonimmunogenic, while those polymers with greater than this number were fully immunogenic. The results lead to the conclusion that the immunological response at its most elementary level is quantized, i.e., a minimum specific number of antigen receptors (approximately 12 to 16) must be connected together as a spatially continuous cluster, an immunon, before an immunogenic signal is delivered to the responding cell.

[Immunosuppressive effect of a human serum ferroprotein of hepatic origin: alpha-2-HF-globulin]

alpha 2HF, a serum ferroprotein increased during some malignant diseases, displays a suppressive effect on in vitro antibody responses, whether T-independent or T-dependent. This effect is not related to the presence of iron, nor to the macroglobulin nature of the molecule. The immunosuppressive properties of alpha 2 HF might depend on its carbohydrate moeity.

In vitro response of mouse spleen cells to the solid phase immunogen DNP-O-Bio-Gel

The in virto immunogenicity of the solid-phase hapten, dinitrophenyl-ornithine-Bio-Gel (DNP-O-Bio-Gel), was investigated in cultures of mouse spleen cells. Appropriate combinations of cells and immobilized hapten were determined. Large numbers of direct anti-hapten plaque-forming cells (PFC) were generated when 1 times 10-7 C57BL/6 or C57BL/10 spleen cells were cultured with 4 times 10-3 DNP-O-Bio-Gel beads. Specificity studies of the responses of cultured spleen cells to DNP-O-Bio-Gel yielded the following results: soluble DNP-ornithine or DNP-bovine gamma-globulin inhibited the induction of anti-hapten PFC by DNP-O-Bio-Gel; neither dinitrophenyl-Bio-gel (DNP-Bio-gel) nor ornithine-Bio-Gel (O-Bio-Gel) induced anti-hapten responsiveness; furthermore, neither DNP-Bio-Gel nor O-Bio-Gel inhibited the induction of PFC by DNP-O-Bio-Gel. It was concluded, from the results of these specificity experiments, that a spacer, ornithine, is required for immunogenicity of immobilized DNP; and that the Bio-Gel bead, itself, acts solely as a physical carrier for the hapten.