Stimulation of the secretory IgA system in monkeys by parenteral immunization with a ribosomal Sonne dysentery vaccine

The experiment was made on 16 monkeys (rhesus macaques). Only 1 out of 12 monkeys immunized with S. sonnei ribosomal vaccine and all 4 control monkeys fell ill as the result of oral challenge with S. sonnei virulent strain. The immunized monkeys stopped excreting Shigellae earlier than the control monkeys. Antibody to lipopolysaccharide (LPS) in the serum and saliva of the monkeys were studied in the enzyme immunoassay with monospecific antibodies to human IgA, IgG and IgM. A single injection of the ribosomal vaccine in a dose of 600 micrograms was shown to lead to a considerable increase in the levels of IgA, IgG and IgM antibodies to LPS in saliva. In parenteral immunization with the ribosomal vaccine the stimulation of secretory IgA system is similar to that resulting from oral challenge with Shigella virulent strain introduced in a dose of 50 X 10(9) microbial cells. No difference in the response of monkeys to primary and booster immunization was noted.     

Organ and isotype distribution of plasma cells producing specific antibody after oral immunization: evidence for a generalized secretory immune system.

Mice were induced to produce IgA antibodies against ferritin after oral immunization. Such antibodies were detected by immunofluorescence in plasma cells in the intestinal mucosa as well as in secretory sites located elsewhere, such as the lactating mammary gland, salivary gland, and respiratory tract. The observation suggested that cells immunized locally via the gut could home to distant secretory sites. To confirm this hypothesis, lymphocyte transfer studies were done with mesenteric node (MN) versus peripheral node (PN) cells from orally immunized donors into nonimmunized recipients. IgA anti-ferritin cells from MN homed to exocrine targets, whereas IgM and IgG anti-ferritin cells homed to PN. The findings overall support the concept of a generalized and interrelated secretory immune system.     

Oral—parenteral immunization leads to the appearance of IgG auto-anti-idiotypic cells in mucosal tissues

The appearance of plasma cells, specific for antigen or autologous idiotypic antibody, in rabbits immunized systemically, orally, or orally-systemically was determined using immunofluorescence techniques. Oral or parenteral immunizations alone did not lead to the appearance of anti-idiotypic cells in mucosal tissues, although IgA antigen-binding cells were observed in these sites after the oral administration of antigen. However, when rabbits were primed with orally administered antigen, followed by a regimen of concurrent oral—parenteral immunizations, IgG anti-idiotypic plasma cells were demonstrated in mucosal lymphoid tissues.     

Murine bone marrow IgA responses to orally administered sheep erythrocytes

Specific immunization protocols have been established for the induction of murine bone marrow IgA responses to the T cell-dependent (TD) antigen sheep red blood cells (SRBC). Systemic immunization, either i.p. or i.v., followed by a second injection, induced splenic IgM and IgG responses and a bone marrow IgM response. No significant IgA responses were observed in either lymphoid tissue compartment. Oral immunization with SRBC by gastric intubation for 2 days, followed 1 wk later by an i.p. injection of SRBC resulted in a splenic IgA plaque-forming cell (PFC) response, but did not elicit a bone marrow IgA response. Repeated daily gastric intubation of SRBC to C3H/HeN and C3H/HeJ mice led to the previously reported pattern of systemic unresponsiveness in C3H/HeN mice and good anamnestic type IgM, IgG, and IgA splenic anti-SRBC PFC responses in the C3H/HeJ strain upon parenteral challenge. Oral administration of SRBC for 14 days to C3H/HeN mice, followed by systemic SRBC challenge, resulted in diminished splenic PFC responses of all isotypes, whereas gastric intubation of SRBC for 28 days led to complete systemic unresponsiveness to antigen in C3H/HeN mice. Interestingly, the repeated oral administration of SRBC resulted in significant bone marrow IgA PFC responses upon i.p. challenge in both C3H/HeN and C3H/HeJ mouse strains. The bone marrow IgA responses were clearly dependent upon chronic oral exposure to SRBC, because gastric intubation with SRBC for 2 consecutive days/wk for 10 wk also induced bone marrow and splenic IgA anti-SRBC PFC responses in C3H/HeN mice. These results suggest that memory B cells reside in the bone marrow of orally immunized mice and can yield anamnestic-type responses to challenge with the inducing antigen. The memory cells may arise in the Peyer's patches of the gut and migrate to the bone marrow. The possibility that the bone marrow is a component of the common mucosal immune system in mammals is suggested by this study.     

Amplification of the secretory IgA response to Toxoplasma gondii using cholera toxin

Our study demonstrates that cholera toxin (CT) markedly enhances the intestinal anti-T. gondii antibody response following oral immunisation of mice with a T. gondii sonicate (TSo) and CT. The antibodies induced were mostly IgA and secretory IgA but a small quantity of IgG was also produced. In contrast, no intestinal anti-T. gondii IgM antibodies were detected. Anti-CT IgA antibodies were also present in intestinal secretions but in much lower quantities than the T. gondii-specific IgA. No anti-CT IgG nor IgM antibodies were detected. Western blot analysis showed that CT induced not only an increase of the intensity of the intestinal IgA antibody response to the 30-kDa band but also induced intestinal IgA antibodies against other major T. gondii proteins (p22, and the 28-kDa antigen) as recognised by specific monoclonal antibodies. The amplification of the anti-T. gondii secretory IgA response by means of an appropriate adjuvant may be one major step leading towards an orally induced immune protection against toxoplasmosis.     

Mucosal immune responses are related to reduction of bacterial colonization in the stomach after therapeutic Helicobacter pylori immunization in mice

The aim of this study was to investigate the capacity of oral and parenteral therapeutic immunization to reduce the bacterial colonization in the stomach after experimental Helicobacter pylori infection, and to evaluate whether any specific immune responses are related to such reduction. C57BL/6 mice were infected with H. pylori and thereafter immunized with H. pylori lysate either orally together with cholera toxin or intraperitoneally (i.p.) together with alum using immunization protocols that previously have provided prophylactic protection. The effect of the immunizations on H. pylori infection was determined by quantitative culture of H. pylori from the mouse stomach. Mucosal and systemic antibody responses were analyzed by ELISA in saponin extracted gastric tissue and serum, respectively, and mucosal CD4+ T cell responses by an antigen specific proliferation assay. Supernatants from the proliferating CD4+ T cells were analyzed for Th1 and Th2 cytokines. The oral, but not the parenteral therapeutic immunization induced significant decrease in H. pylori colonization compared to control infected mice. The oral immunization resulted in markedly elevated levels of serum IgG+M as well as gastric IgA antibodies against H. pylori antigen and also increased H. pylori specific mucosal CD4+ T cell proliferation with a Th1 cytokine profile. Although the parenteral immunization induced dramatic increases in H. pylori specific serum antibody titers, no increases in mucosal antibody or cellular immune responses were observed after the i.p. immunization compared to control infected mice. These findings suggest that H. pylori specific mucosal immune responses with a Th1 profile may provide therapeutic protection against H. pylori.     

Immunogenicity of Vibrio cholerae O1 Fimbriae in Animal and Human Cholera

Parenteral immunization with either formalin-fixed whole cells of the fimbriate Bgd17 strain or purified fimbriae protected against Vibrio cholerae O1 infection in rabbits, independent of biotype and serotype. Parenteral immunization of adult rabbits with purified fimbriae prior to V. cholerae O1 challenge resulted in a reduction of 2 to 3 orders of magnitude in the number of bacteria recovered from the small intestines of immunized rabbits in comparison to non-immunized controls. IgG and IgA antibodies against fimbrillin of V. cholerae O1 were detected in the convalescent sera of patients with cholera; however, little fimbrial antigen was detected in the commercially available cholera vaccines when examined by polyclonal and monoclonal antibodies against fimbriae. These data suggest that fimbrial hemagglutinin is a major adhesin of V. cholerae O1 and that parenteral immunization with fimbriae generates a specific immune response in the gut that may serve as one means of mitigating subsequent V. cholerae O1 gut infection.     

The effect of splenectomy on the immune responses of rabbits to a soluble protein antigen given parenterally or orally

Splenectomized and sham-operated rabbits were immunized with bovine serum albumin (BSA) intravenously, subcutaneously, or orally. Splenectomy caused a 2-to 4-day delay in antibody synthesis in animals immunized intravenously; this delay corresponded to the time that antibody-producing cells were found mainly in the spleen. By 14 days, the antibody response of the splenectomized intravenously immunized group was similar to that of the sham-operated group. Splenectomy did not diminish the antibody responses of rabbits immunized subcutaneously or orally.Splenectomy had no effect on the capacity of circulating lymphocytes to respond to phytohemagglutinin. Similarly, splenectomy did not alter the capacity of circulating lymphocytes from subcutaneously immunized rabbits to respond to BSA in vitro. In contrast, the presence of detectable circulating antigen-reactive lymphocytes in splenectomized animals was slightly reduced after intravenous immunization, and significantly enhanced after oral immunization.Thus, the spleen of the rabbit is important for the early antibody response to soluble protein antigens given intravenously. These studies suggest that the systemic immunity which follows local antigen stimulation at mucosal surfaces, i.e., oral immunization, may be secondary to the circulation of lymphoid cells sensitized in the lamina propria of the intestine.     

Gut-associated lymphoid tissue as source of an IgA immune response in respiratory tissues after oral immunization and intrabronchial challenge

Most IgA plasma cells in the digestive tract are thought to derive from gut-associated lymphoid tissue, whereas IgA plasma cells in the respiratory mucosa are thought to originate largely in bronchus-associated lymphoid tissue. However, previous work has also shown that IgA antibodies to gut antigens can be detected in immunocytes of the bronchial mucosa and in bronchial secretions after appropriate stimulation via the gut. To analyze the cellular origin of such respiratory antibodies, mice were orally immunized with ferritin for 40 days and then segregated for intrabronchial challenge as follows: one group was given saline, another group Formalin-fixed Escherichia coli as a nonspecific challenge, and a third group ferritin. Lungs and intestines from these animals were then examined by immunofluorescence for the presence of plasma cells containing particular isotypes of antibody to ferritin. Animals fed ferritin and given saline or E. coli intrabronchially showed a greater than 6-fold increment in IgA antiferritin plasma cells in the bronchial mucosa, compared to animals which had not received ferritin, whereas orally immunized animals challenged intrabronchially with ferritin showed a greater than 15-fold increase. In other experiments, ferritin-naive animals transfused with mesenteric node cells that were obtained from donors that had been orally immunized with ferritin and were already committed to IgA production showed a 4-fold or greater increase in IgA antiferritin plasma cells in respiratory mucosa after intrabronchial challenge with ferritin when compared to recipients of peripheral node cells from the same donors or to recipients of mesenteric node cells that had not been specifically boosted intrabronchially. These results suggest that immunologically specific IgA immunocytes from gut-associated lymphoid tissue can migrate to the respiratory mucosa after oral immunization, and that migration and/or local cell division are enhanced by subsequent intrabronchial challenge. In providing further evidence for interrelations between gut-associated and bronchus-associated lymphoid tissue, the findings lend added support to the overall concept of a generalized secretory immune system.     

Induction of intestinal IgA and IgG antibodies preventing adhesion of verotoxin-producing Escherichia coli to Caco-2 cells by oral immunization with liposomes

AIMS: To examine the efficacy of liposome oral administration to induce systemic and mucosal immune responses against verotoxin-producing Escherichia coli (VTEC) and the effect of the induced antibodies on the binding of the bacteria to Caco-2 cells. METHODS AND RESULTS: Mice were immunized orally with VTEC antigen and monophosphoryl lipid A (MPL)-containing liposomes composed of dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylserine and cholesterol (1 : 1 : 2, molar ratio) (PS-liposome). After immunization, significant IgA and IgG responses to VTEC were induced in both serum and the intestinal lavage fluid in all mice tested. Furthermore, anti-VTEC IgA and IgG antibodies in the lavage fluid effectively inhibited the adhesion of VTEC to Caco-2 cells. CONCLUSIONS: Oral immunization with liposome-associated E. coli O157:H7 antigen can induce significant systemic and mucosal antibody responses against the bacterial antigen and antibodies produced in the intestinal tract, thus functioning as inhibitors for preventing VTEC infection. SIGNIFICANCE AND IMPACT OF THE STUDY: Oral PS-liposome vaccines containing MPL have the potential usefulness for the induction of a protective mucosal immune response against intestinal diseases.     

Nasopharyngeal-associated lymphoreticular tissue (NALT) immunity: fimbriae-specific Th1 and Th2 cell-regulated IgA responses for the inhibition of bacterial attachment to epithelial cells and subsequent inflammatory cytokine production

To investigate the antibacterial activity of mucosal Th1 and Th2 immune responses induced nasally and orally, mice were immunized with mucosal vaccine containing fimbrial protein of Porphyromonas gingivalis, a causative agent for a destructive chronic inflammation in the periodontium, and cholera toxin (CT) as mucosal adjuvant. Nasal vaccine containing low doses of fimbriae (10 micrograms) and CT (1 microgram) induced Ag-specific Th1/Th2-type response in CD4+ T cells in mucosal effector tissues, including nasal passage and submandibular glands, which accounted for the generation of Ag-specific IgA-producing cells. In contrast, oral immunization required higher amounts of fimbriae and CT for the induction of Ag-specific IgA responses. Fimbriae-specific IgA mAbs generated from submandibular glands of nasally immunized mice inhibited P. gingivalis attachment to and reduced subsequent inflammatory cytokine production from epithelial cells. These findings suggest that nasal vaccination is an effective immunization regimen for the induction of Ag-specific Th1 and Th2 cell-driven IgA immune responses that possess the ability to inhibit bacterial attachment to epithelial cells and subsequent inflammatory cytokine production.     

Oral tolerance revisited: prior oral tolerization abrogates cholera toxin-induced mucosal IgA responses

Oral delivery of a large dose or prolonged feeding of protein Ags induce systemic unresponsiveness most often characterized as reduced IgG and IgE Ab- and Ag-specific CD4(+) T cell responses. It remains controversial whether oral tolerance extends to diminished mucosal IgA responses in the gastrointestinal tract. To address this issue, mice were given a high oral dose of OVA or PBS and then orally immunized with OVA and cholera toxin as mucosal adjuvant, and both systemic and mucosal immune responses were assessed. OVA-specific serum IgG and IgA and mucosal IgA Ab levels were markedly reduced in mice given OVA orally compared with mice fed PBS. Furthermore, when OVA-specific Ab-forming cells (AFCs) in both systemic and mucosa-associated tissues were examined, IgG AFCs in the spleen and IgA AFCs in the gastrointestinal tract lamina propria of mice given OVA orally were dramatically decreased. Furthermore, marked reductions in OVA-specific CD4(+) T cell proliferative and cytokine responses in spleen and Peyer's patches were seen in mice given oral OVA but were unaffected in PBS-fed mice. We conclude that high oral doses of protein induce both mucosal and systemic unresponsiveness and that use of mucosal adjuvants that induce both parenteral and mucosal immunity may be a better way to assess oral tolerance.     

Nasal Immunization of Mice with Human Papillomavirus Type 16 Virus-Like Particles Elicits Neutralizing Antibodies in Mucosal Secretions

To specifically induce a mucosal antibody response to purified human papillomavirus type 16 (HPV16) virus-like particles (VLP), we immunized female BALB/c mice orally, intranasally, and/or parenterally and evaluated cholera toxin (CT) as a mucosal adjuvant. Anti-HPV16 VLP immunoglobulin G (IgG) and IgA titers in serum, saliva, and genital secretions were measured by enzyme-linked immunosorbent assay (ELISA). Systemic immunizations alone induced HPV16 VLP-specific IgG in serum and, to a lesser extent, in genital secretions but no secretory IgA. Oral immunization, even in the presence of CT, was inefficient. However, three nasal immunizations with 5 μg of VLP given at weekly intervals to anesthetized mice induced high (>10⁴) and long-lasting (>15 weeks) titers of anti-HPV16 VLP antibodies in all samples, including IgA and IgG in saliva and genital secretions. CT enhanced the VLP-specific antibody response 10-fold in serum and to a lesser extent in saliva and genital secretions. Nasal immunization of conscious mice compared to anesthetized mice was inefficient and correlated with the absence of uptake of a marker into the lung. However, a 1-μg VLP systemic priming followed by two 5-μg VLP intranasal boosts in conscious mice induced both HPV16 VLP-specific IgG and IgA in secretions, although the titers were lower than in anesthetized mice given three intranasal immunizations. Antibodies in serum, saliva, and genital secretions of immunized mice were strongly neutralizing in vitro (50% neutralization with ELISA titers of 65 to 125). The mucosal and systemic/mucosal HPV16 VLP immunization protocols that induced significant titers of neutralizing IgG and secretory IgA in mucosal secretions in mice may be relevant to genital HPV VLP-based human vaccine trials.     

Antibody formation in young rabbits immunized with heterologous serum proteins

Rabbits were immunized with human or bovine albumin at different intervals after birth and antibody formation was studied by haemagglutination of red cells sensitized with the relevant antigen. The intraperitoneal injection of antigen in amounts of 5 mg. induced antibody formation in some litters 16–20 days after immunization, if the animals were over three days old when immunized. In younger rabbits the same dose induced tolerance. Even when different methods of enhancing the effect of the antigen (Freund’s adjuvant, Al (OH)₃, antigen-conjugated red cells, immune precipitates) or very small doses of antigen were used, antibody formation was still not detected before the 20th day of life. The use of¹³¹I-BSA did not demonstrate the immune phase of elimination of the antigen during 17 days after administration of the antigen, even in rabbits immunized 14 days after birth. The relationship of antibody formation to the induction of tolerance and the difference in the response of newborn rabbits to immunization with the different types of antigen is discussed.     

IgA and IgA diphtheria antitoxin responses from human tonsil lymphocytes.

Human tonsil lymphocytes were stimulated with diphtheria toxoid and then cultured in a Marbrook culture system so that antibodies could be measured in the culture supernatant. Specific antibodies were measured with excess radiolabeled antigen and antisera specific for each immunoglobulin class. Good IgG and IgA diphtheria antitoxin responses have been obtained and responding culture supernatants were shown to neutralize toxin. The relationship between antitoxin response in vitro and immunization of donors with toxoid was investigated. It was found that at least two immunizations after the age of 6 months were necessary to prime the tonsils for an in vitro antibody response. The IgG and IgA in culture supernatants were demonstrated by immunodiffusion and were measured by radioimmunoassay. By sucrose density gradient ultracentrifugation, it was shown that 40% of the IgA produced in the cultures was greater than 7S. Evidence was obtained that neither the IgA nor the specific IgA antitoxin bears secretory piece. It appears that human lymphocytes from tonsils produce polymer IgA in vitro without secretory piece.     

Systemic immunization with pneumococcal polysaccharide vaccine induces a predominant IgA2 response of peripheral blood lymphocytes and increases of both serum and secretory anti-pneumococcal antibodies

Ig class-, and IgA and IgG subclass-specific immune responses to a 23 valent pneumococcal polysaccharide vaccine were studied at a single-cell level in the peripheral blood of systemically immunized adults. With a solid phase enzyme-linked immunospot (ELISPOT) assay, PBMC from immunized individuals were assayed for spontaneous Ag-specific antibody (Ab) production before, and on days 7, 14, and 28 after vaccination. On the day of immunization, no spontaneous Ag-specific Ab-secreting cells could be detected. On day 7 after vaccination, a high frequency of cells secreting Ab specific for pneumococcal polysaccharides (PPS) was observed. The IgA class comprised 79% (geometric mean) of the Ag-specific Ab-secreting cells, whereas IgG- and IgM-secreting cells accounted for 12% and 9%, respectively. The majority of Ag-specific IgA-secreting cells produced Ab of the IgA2 isotype. Serum, saliva, and tears collected before and on days 7, 14, and 28 after vaccination were assayed for specific Ab to the vaccine (anti-PPS Ab) by an ELISA. Serum IgA anti-PPS Ab showed the highest increase after vaccination with a 19-fold increase (geometric mean) which peaked on day 14. However, the ratio of Ag-specific polymeric vs monomeric IgA did not change after immunization. Serum IgG and IgM anti-PPS Ab displayed mean increases of 5.5-fold and 5.6-fold, respectively, on day 14. The most pronounced increase of salivary anti-PPS Ab was observed in the IgG class (4.5-fold on day 28) followed by IgM (4-fold on day 28), IgA (2.0-fold on day 14), IgA1 (2.4-fold on day 14) and IgA2 (2.0-fold on day 14). The levels of total IgA, IgG, and IgM in saliva did not change significantly throughout the course of immunization. IgG and IgM anti-PPS Ab levels in tears increased less than in saliva, whereas IgA behaved similarly as in saliva. There were no significant differences in the Ag-specific increase rates between the IgA, IgG, and IgM isotypes in tears.     

Combined oral/nasal immunization protects mice from Sendai virus infection

Based on the concept of a common mucosal immune system wherein mucosal associated lymphocytes traffic among the various mucous membranes, the murine gastrointestinal tract was immunized with Sendai virus antigens in order to elicit a virus-specific immune response in the respiratory tract. Multiple intragastric (oral) administration of live or killed Sendai virus induced IgA and IgG antiviral antibodies in both gastrointestinal secretions and serum. When cholera toxin as an adjuvant was included along with virus, gut IgA and IgG as well as serum IgA responses were enhanced. Antiviral antibodies induced in respiratory secretions by oral killed virus plus cholera toxin, however, were variable and protection from virus challenge was not demonstrated. Significantly higher levels of respiratory antiviral antibodies were induced if immunization with oral killed Sendai virus/cholera toxin was combined with intranasal administration of small amounts of killed virus. The combined immunization also resulted in protection of both the upper and lower respiratory tracts from virus infection. Protection of the upper respiratory tract was correlated with the presence of IgA antiviral antibodies in nasal washings. On the other hand, protection of the lower respiratory tract was correlated with IgG antiviral antibodies in bronchoalveolar lavage fluids. Immunization with intranasal killed virus alone conferred partial protection to the lower respiratory tract and no protection to the upper respiratory tract. Thus, oral immunization with killed virus antigen could prime for a protective immune response in the murine respiratory tract and this protective response included IgA antibodies.     

Novel adjuvant based on a proteoliposome-derived cochleate structure containing native lipopolysaccharide as a pathogen-associated molecular pattern

Proteoliposomes (PL) from Neisseria meningitidis B have been widely used as a core antigen for antimeningococcal vaccination. PL contain major outer membrane proteins, LPS and phospholipids, and they induce a strong Th1 immune response, but they have low stability in solution. Attending to the need for new vaccine adjuvants, we developed a highly stable cochleate structure (CS) from PL using a technology that allows easy incorporation of new antigens. We explored the ability of PLCS to activate the immune system and its possible application as an adjuvant for parenteral and mucosal routes. Our results showed that PLCS were able to upregulate the expression of MHC class II and costimulatory molecules on human dendritic cells, as well as being able to stimulate the production of soluble mediators of a Th1 response, such as IL-12 and nitric oxide. High levels of anti-PL IgG were detected in serum after i.m. or mucosal (oral and nasal) administration, but also anti-PL secretory IgA was produced in saliva following nasal delivery. The immune response polarization to a Th1 pattern was confirmed by the induction of IgG2a antibodies, positive delayed type hypersensitivity reactions, and IFN-gamma production by splenocytes from immunized mice. The adjuvant potential was explored using PLCS containing ovalbumin (Ova). PLCS-Ova was able to elicit a substantial increase in anti-Ova IgG compared with Ova alone. In addition, a significant reduction in lesion size was observed in mice immunized with Leishmania major antigens in PLCS after challenge with virulent protozoa, suggesting at least partial modulation of the Th2 environment induced by this parasite. In conclusion, our results support the use of PLCS as a potent Th1 adjuvant for parenteral and mucosal vaccines.     

Facilitated production of secretory IgA against Shiga toxin B subunits by intranasal application of antigen-coated polystyrene microspheres

We examined the effects of microspheres as antigen carriers in mucosal immunization. Shiga toxin B subunits (Stx1B) were adsorbed on 6 mum polystyrene microspheres, which were then intranasally administered to mice together with cholera toxin (CT). Stx1B-specific serum IgG production and secretory IgA production at local mucosal sites were enhanced by the use of microspheres. When OVA was used as a model antigen, secretory IgA production but not serum IgG production was enhanced on the use of microspheres. These results indicated that microspheres provide a useful means of potentiating the immune response against Stx1B with weak immunogenicity.     

Detection of HIV-Gag p24-Specific Antibodies in Sera and Saliva of HIV-1-Infected Adults and in Sera of Infants Born to HIV-1-Infected Mothers

Secretory immunoglobulin A (IgA) is known to play an important role in the mucosal defense against a variety of pathogens. Although the role of IgA antibodies during sexual transmission of HIV is not clear, HIV-specific IgA antibodies have been detected in various mucosal secretions of HIV-infected individuals. Using a monoclonal antibody against human IgA, we established an ELISA system to detect anti-HIV p24 IgA antibodies in sera and saliva. We have analyzed the levels of anti-HIV p24 IgG and IgA antibodies in sera and saliva of 107 and 119 adults, respectively, with HIV infection at different clinical stages, and in the sera of 13 infants born to HIV-infected mothers. The level of anti-HIV p24 IgA antibodies was lower in sera and higher in saliva as compared to that of anti-HIV p24 IgG antibodies. Where the percentage of HIV-specific serum antibody-positive cases decreased with disease progression, that of saliva antibody-positive cases increased in AIDS patients. Among the 13 infants born to HIV-infected mothers, 7 infants were HIV-p24-specific serum IgA positive. These sera were negative for anti-HIV p24 secretory IgA, suggesting that some infants develop their own immune responses against HIV infection. Thus, the detection of HIV-specific IgA antibodies, especially in saliva, could be a simple and reliable test for the diagnosis of HIV infection.