Nucleoside specificity in the carrier IgG-dependent induction of tolerance.

Induction of tolerance to nucleoside haptens in BALB/c mice with isologous IgG conjugates bearing four nucleosides simultaneously (A, G, C, T)-IgG was confirmed. A mixture of separate nucleoside-IgG tolerogens (A-IgG, G-IgG, C-IgG, and T-IgG) was as effective or more effective that the (A, G, C,T)-IgG form in suppressing the response to (A, G, C, T)-KLH. The nucleosides acted independently and simultaneously, since tolerogens with varying combinations of nucleosides caused specific suppression of the respones to only those nucleosides present on the tolerogen. Nucleoside-IgG conjugates did not suppress the response to denatured DNA-methylated bovine serum albumin, in which larger oligonucleotide determinants predominate. In varying combinations, guanosine was the dominant nucleoside both for immunization and for induction of tolerance. After three or four immunizations, control immunized animals made mainly IgG anti-nucleoside antibodies and this IgG antibody formation was preferentially suppressed in tolerogen-treated animals. Tolerance could be established before the primary or secondary immunization and it then persisted for at least 75 days through a fourth course of immunization. The same dosage of tolerogen did not reverse a strongly established anti-nucleoside antibody production after a tertiary response.     

Hapten-IgG-induced suppression of the in vitro antibody response: role of hapten density and of antigenic challenge.

We have studied the suppression of the in vitro antibody response of mouse spleen cells to the trinitrophenyl (TNP) hapten by conjugates of TNP-human IgG (TNP-HGG). Normal mouse spleen cells were incubated with conjugates of HGG and of HGG fragments, with different hapten densities. They were then challenged with either a T-dependent or a T-independent antigen. Highly substituted (12 mol of TNP/mol of HGG) conjugates induced a dose-dependent suppression, apparent after short-term incubation at 4 °C, of both T-dependent and T-independent responses. Conjugates of Fab′₂ and Fab′ were as suppressive as conjugates of IgG, whereas conjugates of albumin, aggregated IgG, and β2 microglobulin lacked suppressive activity. In contrast, the lightly substituted conjugates TNP₈-HGG and TNP₂-Fab′₂ induced a time-dependent suppression, affecting only the T-dependent response. This suggests that the suppressive effect of hapten-IgG conjugates is mediated by two different mechanisms according to the density of hapten on the IgG carrier. When these conjugates are used as tolerogens in the in vivo situation, both mechanisms would operate to a variable extent, and this could account for the remarkable tolerogenic properties of hapten-IgG conjugates.     

Antigen recognition in delayed-type hypersensitivity.

It is still uncertain if cell-mediated immune reactions are more or less specific than antibody-mediated reactions. Accordingly, hapten and carrier specificity were examined in delayed hypersensitivity in guinea pigs. Hapten specificity was demonstrated with 2,4-dinitrophenyl (DNP)-guinea pig albumin (GPA), 2,6-DNP-GPA, 2,4,6-trinitrophenyl (TNP)-GPA, and dansyl (DNS)-GPA. Guinea pigs immunized with each of these conjugates were tested 7 days later with the immunogen and the other conjugates. Strong delayed skin responses were highly specific for the immunogen; there were some weak cross-reactions among the nitrophenyl conjugates, no crossre-actions between the DNS and nitrophenyl conjugates, and no responses to unconjugated GPA. Conjugates carrying different numbers (1–45) of 2,4-DNP groups per molecule were all able to elicit specific responses to 2,4-DNP.Carrier specificity in delayed hypersensitivity was confirmed by immunizing with 2,4-DNP-GPA, and challenging with the immunogen, with 2,4-DNP coupled to bovine albumin (BSA), rabbit IgG, ovalbumin, and hemocyanin. Strong responses were seen to the immunogen, a weak response to 2,4-DNP-BSA, and no response to the other conjugates. Specific immune recognition of both hapten and carrier determinants is therefore required for expression of delayed hypersensitivity. These cell-mediated reactions thus appear to be more specific than those of antibody-mediated reactions in solution.     

Ascaris suum: Immunosuppression in mice during acute infection

Mice inoculated by stomach intubation with 10,000 embryonated Ascaris suum eggs, 4, 11, or 21 days before an intraperitoneal (ip) immunization with 2 × 10⁸ sheep erythrocytes (SRBC) had reduced numbers of direct (IgM) splenic hemolytic plaques measured at 4 days after immunization and only a marginal reduction in indirect plaques (IgG) measured at 9 days after immunization. Lower dosages of Ascaris eggs or simultaneous inoculation of Ascaris eggs and SRBC did not suppress antibody responses to SRBC. No reduction in a secondary antibody response to SRBC injected 4 days after Ascaris inoculation was observed. IgM and IgG hemagglutinin titers, as distinguished by 2-mercaptoethanol sensitivity, were suppressed in mice injected ip with 10⁸ SRBC 10 days following inoculation of 10,000 Ascaris eggs, but titers in both Ig classes were similar in infected and control mice injected with 2 × 10⁹ SRBC. At Day 20, antibody titers following ip injection of 1.0 or 100 μg of ovalbumin in alum were reduced in mice infected with 10,000 Ascaris eggs 4 days before antigen injection.Contact hypersensitivity to oxazalone was not altered in mice sensitized at 5 or 14 days after inoculation of 10,000 Ascaris eggs. The delayed hypersensitivity response, measured by footpad swelling, to an optimum intravenous sensitizing dosage of SRBC was inhibited in mice sensitized 10 days after Ascaris infection, but not inhibited in mice sensitized at 21 or 32 days after infection. In contrast, the delayed hypersensitivity response to subcutaneous sensitization with SRBC 10 days after Ascaris infection was not altered.     

B- and T-cell tolerance induction in young-adult and old mice.

Young-adult and old (CS7BL × C3H)F₁ mice were injected with either of two tolerogens followed by challenge with dinitrophenylated human γ globulin (DNP-HGG). The primary IgM and IgG responses of spleen cells to the DNP determinant were evaluated by a modified hemolytic plaque assay. Carrier-specific thymus-derived cell (T-cell) tolerance was induced in mice with deaggregated HGG. Hapten-specific bursal-derived cell (B-cell) tolerance was induced with DNP coupled to isogeneic mouse IgG. The dose of these two tolerogens was successively decreased by 10-fold decrements until a response similar to that of control mice was achieved. The minimum tolerizing dose (MTD) was then determined for young-adult and aged B-and T-cells. We found that the MTD for old B-cells was 10 times greater than that obtained with young B-cells for both the direct and indirect PFC responses. No difference in MTD was observed between young and old T-cells when assessed by the indirect response; the MTD for old T-cells was 10-fold greater than that obtained for young-adult T-cells when the direct response was evaluated. A double threshold of tolerance was found for T-cells of young-adult mice.     

Hymenolepis nana: adoptive transfer of protective immunity and delayed type hypersensitivity response with mesenteric lymph node cells in mice

A marked degree of footpad swelling was observed in BALB/c mice infected with Hymenolepis nana eggs, when soluble egg antigen was injected into their footpads 4 to 21 days after the egg infection, indicating delayed type hypersensitivity responses in infected mice. Adoptive transfer with mesenteric lymph node cells from donor mice (BALB/c strain; +/+) infected with eggs 4 days before cell collection could confer this hypersensitivity to recipient nude mice (BALB/c strain; nu/nu). These mesenteric lymph node cells were then divided into two fractions, blast-enriched and blast-depleted cells, by density gradient centrifugation with Percoll. The recipients intravenously injected with the blast-depleted cell fraction showed a marked increase in footpad thickness, whereas the intravenous transfer of the blast-enriched cell fraction resulted in an insignificant increase in footpad thickness. The transfer of the blast-enriched cell fraction, but not of the blast-depleted cell fraction, conferred a strong adoptive immunity on syngeneic recipient nude mice, when the immunity transferred was assessed by examining cysticercoids developed in the intestinal villi on Day 4 of challenge infection. The lack of delayed type hypersensitivity response in mice that received the blast-enriched cell population was not due to a lack of the capacity of the cells to induce the response, because the cells were capable of inducing a significant increase in thickness of footpads of normal mice when these cells were locally injected into the footpad together with soluble egg antigen.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antitumor properties of vindesine-monoclonal antibody conjugates

Summary The anticancer alkaloid vindesine (VDS) was conjugated to four mouse monoclonal antibodies recognizing human tumor-associated antigens. The antibodies were 96.5 (antimelanoma, IgG2a); 791T/36 (antiosteogenic sarcoma, IgG2b); 11.285.14, and 14.95.55 (anticarcinoembryonic antigen, IgG1 and IgG2a respectively). Conjugates VDS-96.5 and VDS-791T/36 were tested in vitro and shown to be specifically cytotoxic for target cells expressing the appropriate antigen. The in vivo effects of the antibodies and conjugates were tested against human tumor xenografts in athymic or immunodeprived mice using multiple treatments. Conjugate VDS-96.5 retarded the initial growth of a melanoma xenograft, whereas free antibody was without effect. Similarly, VDS-791T/36 but not free antibody retarded the growth of osteogenic sarcoma 791T. The most marked antitumor effects observed were those obtained with VDS conjugates of the anti-CEA antibodies against a colorectal tumor xenograft. Antibody 14.95.55 suppressed tumor growth both alone and as a VDS conjugate, whereas 11.285.14 produced only a slight effect alone but an almost complete and lasting suppression of tumor growth as a VDS conjugate. Free VDS had little effect at nontoxic levels. Acute studies showed that VDS-11.285.14 conjugate was considerably less toxic than free VDS in Balb/c mice.     

Relationships between cell-mediated immunity and the IgE antibody response: II. Delayed hypersensitivity and antibody production to DNP-Ascaris conjugates

Rats of the W/F strain were immunized with DNP-Ascaris conjugates using complete Freund's adjuvant (CFA), Al(OH)₃ gel (alum), or B. pertussis vaccine as adjuvants. Cell-mediated immunity was assessed by lymphotoxin in vitro and by delayed hypersensitivity in vivo. IgE and IgG antibody determinations were made on serum pools obtained at various times during the primary and secondary responses. Although delayed hypersensitivity appeared earlier than lymphotoxin, these two parameters correlated during the primary but not during the secondary response. The discrepancies suggested that different cells may be responsible for these two phenomena. Antibody production was influenced by the adjuvant used. CFA led to IgG antibody responses to both hapten and carrier but not to IgE antibody production. The use of B. pertussis resulted in both IgE and IgG antibody production. In the case of alum, anti-hapten antibodies appeared during the primary response while anti-carrier antibodies of both IgE and IgG classes were detected after booster. The results indicated that cell-mediated immunity, IgE, and IgG antibodies appeared independently in an ordered, temporal sequence, and that these responses were not mutually exclusive but were under strong modulatory influences of the various adjuvants used.     

In vivo and in vitro effects of monoclonal antibody to Ly antigens on immunity to infection

Injection of CBA mice with Brucella abortus strain 19 leads to chronic infection during which both cell-mediated immunity (delayed hypersensitivity and macrophage activation) and antibody production occur. Protection was efficiently transferred to naive mice using spleen cells from mice infected 5 or 12 weeks earlier. Selective lysis in vitro of these cells by antibody to cell surface antigens showed that Thy-1⁺ Ly-1⁺2⁺ T lymphocytes were required for transfer. Treatment with anti-Ia serum neither suppressed nor enhanced adoptive transfer. Thus Ia⁺ B lymphocytes were not required, and Ia⁺ suppressor T cells were not active in the response. Three injections per week of anti-Ly-1 monoclonal antibody beginning 5 days before infection led to a 10-fold increase in bacterial numbers 25 days after infection when acquired immunity was well established in untreated mice. The delayed hypersensitivity response was unaffected. In addition cells from these in vivo treated mice were unable to transfer resistance. Beginning the treatment on the day of infection abolished the IgG antibody response without affecting bacterial numbers. The studies emphasize the unique role of Ly-1⁺2⁺ T cells in immunity to Brucella and indicate the usefulness of these techniques in dissecting out those components of the immune response which contribute to recovery from infection.     

Transmission of hapten-specific tolerance to an unrelated carrier

The principle of linked recognition is well defined in response and suppression. Yet, to our knowledge, it is not explored in the context of tolerance. To investigate, whether the status of tolerance toward a hapten (TNP) can be transferred to a subsequently introduced carrier, animals which were tolerized by a subimmunogenic dose of hapten (TNP) coupled to syngeneic monoclonal anti-TNP IgG, with the rationale of combining the phenomena of low zone tolerance and syngeneic IgG-induced suppression, were challenged with TNP-horse red blood cells (HRBC). Conjugates of high density (40 mM) TNP-syngeneic IgG (TNP40-IgG) were immunogenic and after challenge with TNP-HRBC, animals responded to TNP and to HRBC. Yet, spleen cells (SC) of mice injected with TNP2.5-IgG and challenged with TNP-HRBC were tolerant against TNP as well as the carrier. Limiting dilution (LD) analysis revealed that subimmunogenic doses of TNP coupled to IgG resulted in diminished activation of help, failure to activate contrasuppressor T cells (TCS), and significantly augmented activation of suppressor T cells (TS). On the other hand, after challenge with TNP-HRBC, activation/expansion of carrier-specific helper (TH), suppressor, and contrasuppressor T cells were not affected by previous immunization with subimmunogenic or immunogenic doses of TNP-IgG conjugates, but HRBC-specific TCS could not interact with TNP-specific TS. Hence, to initiate tolerance it was necessary (and sufficient) that an activated and expanded TS population was not counterregulated by TCS. In this situation, an established status of dominance of suppression for the epitope TNP could not be disrupted by an immunogene carrying a multitude of new epitopes; i.e., tolerization by subimmunogenic doses of the individual epitope TNP resulted in unresponsiveness against any immunogen carrying this epitope.     

Successful immunization against experimental allergic encephalomyelitis with myelin basic protein-sensitized allogeneic lymphocytes

Prevention and suppression of experimental allergic encephalomyelitis were demonstrated in rats, guinea pigs, and rabbits immunized with allogeneic, but not with syngeneic lymphocytes from susceptible donors sensitized to myelin basic protein (MBP). Donor lymphnode, splenic, or peripheral blood lymphocytes were effective in inducing a state of unresponsiveness to an encephalitogenic challenge in either of the three species. Unresponsiveness was not obtained in recipients immunized with sensitized allogenic lymphocytes and simulatenously challenged with MBP suggesting that a time lapse between immunization and challenge is necessary for the development of protective immunity. Induced in immunized recipients, unresponsiveness was transferred into normal syngeneic recipients with immunoglobulin-G (IgG) isolated from protected donors before challenge. Furthermore, both immunized and IgG recipients failed to develop cell-mediated immunity after challenge with MBP. The results show that prevention and suppression of EAE was mediated by antibodies which inhibited the development of delayed type hypersensitivity to the challenging antigen.     

肺炎球菌表面蛋白A(PspA)及其荚膜多糖交联物的免疫原性和交叉保护作用

分别采用肺炎球菌表面蛋白A(PspA)和PspA-荚膜多糖交联物免疫小鼠,研究PspA及其交联物的免疫特性.用酶联免疫吸附法(ELISA)检测抗原的免疫原性,用动物保护试验检验抗原对肺炎球菌6B,5,1,23F,19F型的交叉免疫效果.实验结果表明:肺炎球菌表面蛋白A及其多糖交联物表现出一定的交叉保护作用,具有较好的应用前景.     

Protection and recovery in influenza virus-infected mice immunosuppressed with anti-IgM

BALB/c mice, immunosuppressed from birth with goat anti-mouse IgM, were able to recover from influenza virus infection in the absence of detectable serum and nasal antibody. Recovery was delayed a few days when compared with control animals. Antibody-deficient mice, that had recovered from an initial influenza virus infection, i.e., convalescent mice, were subsequently rechallenged with homologous influenza virus in order to study the importance of nasal and serum antibody in prevention of infection. Convalescent mice were susceptible to reinfection when nasal and serum antibody were not detectable. The mice were resistant to reinfection when serum and/or nasal antibody was detectable by radioimmunoassay. Normal mice that were passively immunized with high titer mouse anti-influenza virus serum were susceptible to challenge with homologous influenza virus. The serum antibody levels in these mice were higher than most of those found in the immune convalescent mice suppressed with anti-IgM, thereby suggesting that the serum antibody, found in convalescent suppressed mice, is not protective. We conclude that 1) mice can recover from influenza virus infection in the absence of detectable levels of nasal and serum antibody, thus indirectly confirming the role of cell-mediated immunity in recovery; 2) serum IgM, IgG2A, IgG2B, IgG3, and probably IgG1 antibody levels are not responsible for protection against influenza virus infection of the upper respiratory tract; and 3) nasal IgA antibody correlates best with protection against reinfection of the upper respiratory tract, but some other locally protective agent cannot be excluded.     

Role of mediators of cellular hypersensitivity in the stimulation of the antibody formation to unrelated antigen

The effect of a concurrent delayed hypersensitivity reaction on the antibody response to sheep red cells was assessed by a plaque assay. Guinea pigs with delayed hypersensitivity to tuberculin purified protein derivative (PPD) or egg albumin showed an increased antibody response to sheep red cells when the cells were injected intravenously at the same time as PPD or egg albumin. This effect was transferred to normal guinea pigs by serum from guinea pigs with delayed hypersensitivity to PPD or egg albumin taken 24 hr after injecting the corresponding antigen. Supernatants containing migratory inhibitory factor were prepared by incubating lymphocytes from sensitized rabbits with antigen. These supernatants were injected with sheep red cells and gave rise to an enhanced plaque response. Similar results were obtained with supernatants from normal rabbit thymus cells. The role of mediators of delayed hypersensitivity in enhancing antibody formation and in T cell/B cell cooperation is discussed.     

Suppression in murine experimental autoimmune thyroiditis: in vivo inhibition of CD4+ T cell-mediated resistance by a nondepleting rat CD4 monoclonal antibody.

Genetically susceptible mice become resistant to experimental autoimmune thyroiditis (EAT) induction with mouse thyroglobulin (MTg) and lipopolysaccharide after pretreatment with deaggregated MTg (dMTg). Recent work showed this suppression to be mediated by CD4+ suppressor T cells (Ts). To study Ts action in vivo, we used a rat IgG2a monoclonal antibody (mAb), YTS 177.9, which modulates CD4 antigen in vivo without depleting CD4+ cells. Initial studies showed that after two 1-mg doses of mAb 7 days apart, extensive CD4 antigen modulation of peripheral blood leukocytes occurred within 4 days. Mice given CD4 mAb 24 hr before dMTg (2 doses, 7 days apart) were resistant to EAT induction when immunized with MTg and LPS 20 days later. Also, anti-rat IgG2a titers were reduced following challenge with heat-aggregated rat IgG2a compared to controls. Subsequent analysis of serum in CD4 mAb-treated animals revealed that mAb was present in the circulation for 14 days. Moreover, mice given CD4 mAb and dMTg, then challenged after only 10 days, when CD4 mAb was still circulating, developed a significantly higher incidence of thyroid damage than controls. These findings suggest that modulation of CD4 antigen does not interfere with Ts activation, but the presence of CD4 mAb, at the time of autoantigenic challenge, can interfere with tolerance to EAT induction. Thus, the direct relationship between the presence of CD4 mAb and inhibition of EAT suppression implicates a role for CD4 molecules in the mediation of suppression.     

Antibodies to guinea pig lymphokines. II. Suppression of delayed hypersensitivity reactions by a "second generation" goat antibody against guinea pig lymphokines.

A "second generation" antibody to a highly purified lymphocyte product was raised in a goat against material eluted from a rabbit anti-guinea pig lymphokine immunoadsorbent column. This anti-lymphokine serum, in constrast to anti-lymphocyte serum (ALS) did not appear to contain cytotoxic antibodies directed against membrane antigens on guinea pig lymph node lymphocytes. Furthermore, the anti-lymphokine serum did not inhibit the formation of spontaneous T rosettes nor significantly depress lymphocyte response to mitogens. The anti-lymphokine serum totally suppressed the delayed skin reactivity to PPD and contact sensitivity to DNCB when injected intradermally around the site of antigen challenge. By contrast, intradermally injected ALS did not appear to suppress the PPD response in sensitized guinea pigs. Intravenously and i.p. administered anti-lymphokine serum was somewhat less effective in suppressing the delayed skin response to PPD. The intradermal injection of the antiserum had no effect on nonspecific inflammation evoked by turpentine-olive oil or on the extravasation of circulating Evans blue evoked by intradermally injected histamine. Histologic examination of 24-hr DNCB-induced skin lesions from sensitized guinea pigs treated with intradermally injected anti-lymphokine serum showed marked reduction of mononuclear infiltration of the dermis and of epidermal lesions, as compared with skin sites taken from sensitized animals pretreated with normal goat serum. The anti-lymphokine serum injected i.v. also markedly reduced the perivascular infiltration of the dermis and subcutis in skin reaction sites from sensitized animals challenged with PPD. Intravenous treatment with ALS for 3 consecutive days caused extensive depletion of the paracortical areas of peripheral lymph nodes whereas treatment with normal serum and anti-lymphokine serum caused no such depletion. It is proposed that the anti-lymphokine serum is directed against activated lymphocyte products, one of them being MIF. These products are involved in the mediation of delayed hypersensitivity reactions. This is in marked contrast to ALS, the suppressive action of which appears to be central rather than peripheral.     

Suppression of cell-mediated immunity by cyclophosphamide: its independence of concomitant B cell response

Cyclophosphamide in a single dose of 300 mg per kg, injected intraperitoneally 3 days before the sensitization of guinea pigs with azobenzenearsonate-N-acetyl-1-tryosine in complete Freund's adjuvant, suppressed the development of delayed hypersensitivity to PPD tuberculin and Ars-insulin. Peritoneal cell migration inhibition induced in vitro by PPD or Ars-Tyr was also suppressed by the Cy pretreatment. It was concluded that Cy suppresses cell-mediated immunity directly, i.e., in a system, in which no B cell response is available as a target for Cy (the response to ArsTyr).     

Immunity to the T1699 murine mammary tumor. I. Thymic influence and long-term effect of irradiation on the humoral response.

The thymus dependency of humoral immunity to syngeneic tumor antigens was investigated in T cell-deficient DBA/2 mice. ATXBM animals bearing the T1699 mammary tumor in the subcutaneous abdominal area displayed normal immediate but not delayed hypersensitivity reactions (DHR) to 3 M KCl-extracted T1699 antigens injected into the footpad. Sera from ATXBM tumor-bearers passively transferred immediate hypersensitivity but failed to support tumor-specific macrophage-mediated ADCC reactions. The synthesis of macrophage-mediated ADCC antibody was greatly reduced in the CXBM animals when compared to nonirradiated tumor-bearers. The CXBM mice, however, showed normal T cell function as measured by allograft rejection, antibody response to sheep erythrocytes, and DHR to T1699 antigens. Of all antibody classes and subclasses tested by indirect membrane fluorescence, only IgG2b was found to be produced at normal levels by either ATXBM or CXBM tumor-bearers. The results show that IgG2b antibody production in response to T1699 syngeneic tumor antigens is thymus independent and suggest that this antibody is the mediator of immediate hypersensitivity. The synthesis of macrophage-mediated ADCC antibody (IgG2a) was found to be not only thymus dependent but also sensitive to the long-term effects of irradiation and bone marrow repopulation.     

Acute and subacute toxicity of chemotactic conjugates between monoclonal antibody and fMet-Leu-Phe in humans: A phase I clinical trial

Summary Conjugates of the chemotactic peptide fMet-Leu-Phe (fMLP) to IgG retain chemotactic and antigen recognition function in vitro and enhance intra-tumour macrophage numbers in a guinea pig model. We report a study approved by the ethics committee on the acute toxicity of fMLP conjugates in ten consenting cancer patients with metastasizing melanoma and colon cancer. They were given increasing single doses (1–2500 µg) IgG-fMLP made with the anti-melanoma monoclonal antibody (mAb) 9.2.27. Clinical examinations and blood cell counts, urinalysis, electrolytes, and liver and kidney function tests before and after the infusion and weekly thereafter revealed no relevant toxicities. One patient had a herpes zooster exacerbation on day 1, which was judged to be coincidental. Peak post-infusion conjugate serum concentrations fell to unmeasurable levels within a few days. In no case was a human humoral anti-(mouse Ig) immune response detected.This work was supported by grant FOR.254.AK.83 of the Swiss Cancer League.     

Brugia malayi: intravenous injection of microfilariae in ferrets as an experimental method for occult filariasis

Microfilaremia, immune responses, and pathology were compared in ferrets infected with 100 third-stage larvae of Brugia malayi (subperiodic strain) or injected intravenously with 10(6) microfilariae. Ferrets (Mustela putorius furo) inoculated with third-stage larvae typically became patent during the third month after infection, with a mean patency of 123 +/- 25 (SE) days. Ferrets injected intravenously with microfilariae exhibited a relatively constant microfilaremia for 3-4 weeks and usually cleared microfilariae before the fourth month. Ferrets that cleared microfilariae after intravenous injection of microfilariae or after infection with third-stage larvae failed to become patent or became amicrofilaremic within 3 weeks after a challenge intravenous injection of 10(6) microfilariae. Clearance of circulating microfilariae was associated with eosinophilia and serum antibody specific for the microfilarial sheath in ferrets injected with microfilariae and in most ferrets infected with third-stage larvae. Ferrets infected with third-stage larvae and necropsied after clearance of microfilariae had tissue inflammatory reactions to microfilariae characteristic of occult filariasis (tropical eosinophilia) in man; these ferrets exhibited immediate cutaneous hypersensitivity and circulating reaginic antibody to antigens of microfilariae. In ferrets necropsied following two intravenous injections of microfilariae, the majority of ferrets examined within 10 days after clearance of microfilariae had visible liver lesions to microfilariae identical to those of the ferrets infected with third-stage larvae; immediate cutaneous hypersensitivity and reaginic antibody were not consistently detected in ferrets injected with microfilariae. Sera from ferrets that had cleared circulating microfilariae were transferred passively into ferrets made microfilaremic by intravenous injection of microfilariae. Sera with microfilarial sheath-reactive IgG antibody titers (greater than or equal to 1:200) and microfilarial agglutination titers (greater than or equal to 1:40) rapidly cleared injected microfilariae (less than 24 hr); this serum also cleared or greatly reduced circulating microfilariae established by an infection with third-stage larvae; only the IgG-containing fraction of the sera was active in immune clearance. Sera that cleared microfilariae of B. malayi did not clear circulating microfilariae of Dirofilaria immitis or prevent recurrence of circulating microfilariae of B. malayi in ferrets infected with adult filariae.(ABSTRACT TRUNCATED AT 400 WORDS)