A phase IIa, randomized, double-blind, safety, immunogenicity and efficacy trial of Plasmodium falciparum vaccine antigens merozoite surface protein 1 and RTS,S formulated with AS02 adjuvant in healthy, malaria-naïve adults

BACKGROUND

To improve the efficacy of Plasmodium falciparum malaria vaccine RTS,S/AS02, we conducted a study in 2001 in healthy, malaria-naïve adults administered RTS,S/AS02 in combination with FMP1, a recombinant merozoite surface-protein-1, C-terminal 42kD fragment.

METHODS

A double-blind Phase I/IIa study randomized N = 60 subjects 1:1:1:1 to one of four groups, N = 15/group, to evaluate safety, immunogenicity, and efficacy of intra-deltoid half-doses of RTS,S/AS02 and FMP1/AS02 administered in the contralateral (RTS,S + FMP1-separate) or same (RTS,S + FMP1-same) sites, or FMP1/AS02 alone (FMP1-alone), or RTS,S/AS02 alone (RTS,S-alone) on a 0-, 1-, 3-month schedule. Subjects receiving three doses of vaccine and non-immunized controls (N = 11) were infected with homologous P. falciparum 3D7 sporozoites by Controlled Human Malaria Infection (CHMI).

RESULTS

Subjects in all vaccination groups experienced mostly mild or moderate local and general adverse events that resolved within eight days. Anti-circumsporozoite antibody levels were lower when FMP1 and RTS,S were co-administered at the same site (35.0 µg/mL: 95 % CI 20.3-63), versus separate arms (57.4 µg/mL: 95 % CI 32.3-102) or RTS,S alone (62.0 µg/mL: 95 % CI: 37.8-101.8). RTS,S-specific lymphoproliferative responses and ex vivo ELISpot CSP-specific interferon-gamma (IFN-γ) responses were indistinguishable among groups receiving RTS,S/AS02. There was no difference in antibody to FMP1 among groups receiving FMP1/AS02. After CHMI, groups immunized with a RTS,S-containing regimen had ∼ 30 % sterile protection against parasitemia, and equivalent delays in time-to-parasitemia. The FMP1/AS02 alone group showed no sterile immunity or delay in parasitemia.

CONCLUSION

Co-administration of RTS,S and FMP1/AS02 reduced anti-RTS,S antibody, but did not affect tolerability, cellular immunity, or efficacy in a stringent CHMI model. Absence of efficacy or delay of patency in the sporozoite challenge model in the FMP1/AS02 group did not rule out efficacy of FMP1/AS02 in an endemic population. However, a Phase IIb trial of FMP1/AS02 in children in malaria-endemic Kenya did not demonstrate efficacy against natural infection.

CLINICALTRIALS

gov identifier: NCT01556945.

Correction: Identification of Novel Pre-Erythrocytic Malaria Antigen Candidates for Combination Vaccines with Circumsporozoite Protein

[This corrects the article DOI: 10.1371/journal.pone.0159449.].

An early commitment to expression of a particular TCRVbeta chain on CD8(+) T cells responding to attenuated Plasmodium berghei sporozoites is maintained following challenge with infectious sporozoites

Protection induced by irradiated Plasmodium berghei sporozoites (Pbgamma-spz) in mice is linked to CD8(+) T cells specific for exo-erythrocytic-stage Ags, and intrahepatic memory CD8(+) T cells are associated with protracted protection. However, the Ag specificity of the protective CD8(+) T cells remains largely unknown. In this study, we characterized the TCR Vbeta usage by intrahepatic CD8(+) T cells during gamma-spz immunization and after the challenge with infectious Pb sporozoites. The repertoire of naïve (T(N)) and central memory (T(CM)) CD8(+) T cells was diverse and conserved between individual mice, and did not change with immunization. In contrast, preferential usage of one or more TCR Vbeta subset was observed in effector memory (T(EM)) CD8(+) T cells after immunization. The expanded TCR Vbeta varied between individual mice but Vbeta4, 6, 7, 8.3, 9 and 11 were the most frequently expressed. In addition, there was a correlation in the TCR Vbeta usage by gamma-spz-induced CD8(+) T(EM) in the liver and blood of individual mice. The expansion pattern of blood CD8(+) T(EM) did not change with challenge and remained the same for 8 weeks thereafter. These results demonstrate that immunization with gamma-spz skews the TCR Vbeta repertoire of CD8(+) T(EM), and commitment to a particular TCR Vbeta expression is maintained long-term.

The dissection of CD8 T cells during liver-stage infection

Multiple injections of gamma-radiation-attenuated Plasmodium sporozoites (gamma-spz) can induce long-lived, sterile immunity against pre-erythrocytic stages of malaria. Malaria antigen (Ag)-specific CD8 T cells that produce IFN-gamma are key effector cells in this model of protection. Although there have been numerous reports dealing with gamma-spz-induced CD8 T cells in the spleen, CD8 T cells most likely confer protection by targeting infected hepatocytes. Consequently, in this chapter we discuss observations and hypotheses concerning CD8 T cell responses that occur in the liver after an encounter with the Plasmodium parasite. Protracted protection against pre-erythrocytic stages requires memory CD8 T cells and we discuss evidence that gamma-spz-induced immunity is indeed accompanied by the presence of intrahepatic CD44hi CD45RBlo CD62lo CD122lo effector memory (EM) CD8 T cells and CD44hi CD45RBhi CD621hi CD122hi central memory (CM) CD8 T cells. In addition, the EM CD8 T cells rapidly release IFN-gamma in response to spz challenge. The possible role of Kupffer cells in the processing of spz Ags and the production of cytokines is also considered. Finally, we discuss evidence that is consistent with a model whereby intrahepatic CM CD8 T cells are maintained by IL-15 mediated-homeostatic proliferation while the EM CD8 T cells are conscripted from the CM pool in response to a persisting depot of liver-stage Ag.

Pre-erythrocytic immunity to Plasmodium falciparum: the case for an LSA-1 vaccine

A vaccine is urgently needed to stem the global resurgence of Plasmodium falciparum malaria. Vaccines targeting the erythrocytic stage are often viewed as an anti-disease strategy. By contrast, infection might be completely averted by a vaccine against the liver stage, a pre-erythrocytic stage during which the parasite multiplies 10000-fold within hepatocytes. Sterilizing immunity can be conferred by inoculating humans with irradiated pre-erythrocytic parasites, and a recombinant pre-erythrocytic vaccine partially protects humans from infection. Liver-stage antigen-1, one of a few proteins known to be expressed by liver-stage parasites, holds particular promise as a vaccine. Studies of naturally exposed populations have consistently related immune responses against this antigen to protection.

Efficacy of recombinant circumsporozoite protein vaccine regimens against experimental Plasmodium falciparum malaria

After initial successful evaluation of the circumsporozoite-based vaccine RTS,S/SBAS2, developed by SmithKline Beecham Biologicals with the Walter Reed Army Institute of Research, protective efficacy of several regimens against Plasmodium falciparum challenge was determined. A controlled phase 1/2a study evaluated 1 or 2 standard doses of RTS,S/SBAS2 in 2 groups whose members received open-label therapy and 3 immunizations in blinded groups who received standard, one-half, or one-fifth doses. RTS,S/SBAS2 was safe and immunogenic in all groups. Of the 41 vaccinees and 23 control subjects who underwent sporozoite challenge, malaria developed in 7 of 10 who received 1 dose, in 7 of 14 who received 2 doses, in 3 of 6 who received 3 standard doses, in 3 of 7 who received 3 one-half doses, in 3 of 4 who received 3 one-fifth doses, and in 22 of 23 control subjects. Overall protective efficacy of RTS,S/SBAS2 was 41% (95% confidence interval, 22%-56%; P=.0006). This and previous studies have shown that 2 or 3 doses of RTS,S/SBAS2 protect against challenge with P. falciparum sporozoites.

The role of intrahepatic lymphocytes in mediating protective immunity induced by attenuated Plasmodium berghei sporozoites

Exposure to irradiated Plasmodium sporozoites (gamma-spz) results in protection against malaria. Like infectious spz, gamma-spz colonize hepatocytes to undergo maturation. Disruption of liver stage development prevents the generation of protection, which appears, therefore, to depend on liver stage antigens. Although some mechanisms of protection have been identified, they do not include a role for intrahepatic mononuclear cells (IHMC). We demonstrated that P. berghei gamma-spz-immune murine IHMC adoptively transfer protection to naive recipients. Characterization of intrahepatic CD4+ T cells revealed an immediate, albeit transient, response to gamma-spz, while the response of CD8+ T cells is delayed until acquisition of protection. It is presumed that activated CD8+ T cells home to the liver to die; gamma-spz-induced CD8+CD45RB(lo)CD44(hi) T cells, however, persist in the liver, but not the spleen, during protracted protection. The association between CD8+CD45RB(lo)CD44(hi) T cells and protection has been verified using MHC class I and CD1 knockout mice and mice with disrupted liver stage parasites. Based on kinetic studies, we propose that interferon-gamma, presumably released by intrahepatic effector CD8+ T cells, mediates protection; the persistence of CD8+ T cells is, in turn, linked to Plasmodium antigen depots and cytokines released by CD4+ T cells and/or NK T cells.

Altered hepatic lymphocyte subpopulations in obesity-related murine fatty livers: potential mechanism for sensitization to liver damage

Although obesity-related fatty livers are vulnerable to damage from endotoxin, the mechanisms involved remain obscure. The purpose of this study was to determine if immunologic priming might be involved by determining if fatty livers resemble normal livers that have been sensitized to endotoxin damage by Propionibacterium acnes infection. The latter induces interleukin (IL)-12 and -18, causing a selective reduction of CD4+NK T cells, diminished IL-4 production, deficient production of T-helper type 2 (Th-2) cytokines (e.g., IL-10), and excessive production of Th-1 cytokines (e.g., interferon gamma [IFN-gamma]). Liver and spleen lymphocyte populations and hepatic cytokine production were compared in genetically obese, ob/ob mice (a model for obesity-related fatty liver) and lean mice. Obese mice have a selective reduction of hepatic CD4+NK T cells. Serum IL-18 is also increased basally, and the hepatic mRNA levels of IL-18 and -12 are greater after endotoxin challenge. Thus, up-regulation of IL-18 and IL-12 in fatty livers may reduce hepatic CD4+NK T cells. In addition, mononuclear cells from fatty livers have decreased expression of the adhesion molecule, leukocyte factor antigen-1 (LFA-1), which is necessary for the hepatic accumulation of CD4+NK T cells. Consistent with reduced numbers of hepatic CD4+NK T cells, mononuclear cells from fatty livers produce less IL-4. Furthermore, after endotoxin treatment, hepatic induction of IL-10 is inhibited, while that of IFN-gamma is enhanced. Thus, fatty livers have inherent immunologic alterations that may predispose them to damage from endotoxin and other insults that induce a proinflammatory cytokine response.

Memory phenotype CD8(+) T cells persist in livers of mice protected against malaria by immunization with attenuated Plasmodium berghei sporozoites

Natural exposure to Plasmodium parasites induces short-lived protective immunity. In contrast, exposure to radiation-attenuated sporozoites (gamma spz) promotes long-lasting protection that is in part mediated by CD8(+) T cells that target exoerythrocytic stage antigens. The mechanisms underlying the maintenance of long-lasting protection are currently unclear. The liver is a repository of Plasmodium antigens and may support the development and / or homing of memory T cells. While activated CD8(+) T cells are presumed to die in the liver, the fate of anti-Plasmodium CD8(+) T cells remains unknown. We propose that inflammatory conditions in the liver caused by Plasmodium parasites may allow some effector CD8(+) T cells to survive and develop into memory cells. To support this hypothesis, in this initial study we demonstrate that liver mononuclear cells from P. berghei gamma spz-immune mice transferred protection to naive recipients and moreover, that CD4(+) and CD8(+) T cells responded to Plasmodium antigens by up-regulating activation / memory markers. While CD4(+) T cells under went a transient activation following immunization with gamma spz, CD8(+) T cells expanded robustly after spz challenge and exhibited stable expression of CD44(hi) and CD45RB(lo) during protracted protection. These results establish a key role for intrahepatic T cells in long-lasting protection against malaria.

Phase I trial of two recombinant vaccines containing the 19kd carboxy terminal fragment of Plasmodium falciparum merozoite surface protein 1 (msp-1(19)) and T helper epitopes of tetanus toxoid

The safety and immunogenicity of 2 yeast-derived, blood-stage malaria vaccines were evaluated in a phase l trial. Healthy adults were given 2 or 3 doses of alum-adsorbed vaccine containing the 19 kDa carboxy-terminal fragment of the merozoite surface protein-1 (MSP-1(19)) derived from the 3D7 or the FVO strain of Plasmodium falciparum fused to tetanus toxoid T-helper epitopes P30 and P2. The first 2 doses of MSP-1(19) were well tolerated. Hypersensitivity reactions occurred in 3 subjects after the third dose of MSP-1(19), including bilateral injection site reactions in 2 (one with generalized skin rash), and probable histamine-associated hypotension in 1. Serum antibody responses to MSP-1(19) occurred in 5/16, 9/16 and 0/8 subjects given 20 microg of MSP-1(19), 200 microg of MSP-1(19), and control vaccines (hepatitis B or Td), respectively. Both MSP-1(19) vaccines were immunogenic in humans, but changes in formulation will be necessary to improve safety and immunogenicity profiles.

Long-term efficacy and immune responses following immunization with the RTS,S malaria vaccine

The malaria sporozoite vaccine candidate RTS,S, formulated with an oil-in-water emulsion plus the immunostimulants monophosphoryl lipid A and the saponin derivative QS21 (vaccine 3), recently showed superior efficacy over two other experimental formulations. Immunized volunteers were followed to determine the duration of protective immune responses. Antibody levels decreased to between one-third and one-half of peak values 6 months after the last dose of vaccine. T cell proliferation and interferon-gamma production in vitro were observed in response to RTS,S or hepatitis B surface antigen. Seven previously protected volunteers received sporozoite challenge, and 2 remained protected (1/1 for vaccine 1, 0/1 for vaccine 2, and 1/5 for vaccine 3). The prepatent period was 10.8 days for the control group and 13.2 days for the vaccinees (P < .01). Immune responses did not correlate with protection. Further optimization in vaccine composition and/or immunization schedule will be required to induce longer-lasting protective immunity.

Phase I/IIa safety, immunogenicity, and efficacy trial of NYVAC-Pf7, a pox-vectored, multiantigen, multistage vaccine candidate for Plasmodium falciparum malaria

Candidate malaria vaccines have failed to elicit consistently protective immune responses against challenge with Plasmodium falciparum. NYVAC-Pf7, a highly attenuated vaccinia virus with 7 P. falciparum genes inserted into its genome, was tested in a phase I/IIa safety, immunogenicity, and efficacy vaccine trial in human volunteers. Malaria genes inserted into the NYVAC genome encoded proteins from all stages of the parasite's life cycle. Volunteers received three immunizations of two different dosages of NYVAC-Pf7. The vaccine was safe and well tolerated but variably immunogenic. While antibody responses were generally poor, cellular immune responses were detected in >90% of the volunteers. Of the 35 volunteers challenged with the bite of 5 P. falciparum-infected Anopheles mosquitoes, 1 was completely protected, and there was a significant delay in time to parasite patency in the groups of volunteers who received either the low or high dose of vaccine compared with control volunteers.

Detection of CD4+CD45RO+ T lymphocytes producing IL-4 in response to antigens on Plasmodium falciparum erythrocytes: an in vitro correlate of protective immunity induced with attenuated Plasmodium falciparum sporozoites

Malaria is caused by Plasmodium spp. and is one of the major infectious diseases leading to morbidity and mortality in tropical areas of the world. The model of protective immunity induced by immunization with radiation-attenuated Plasmodia sporozoites (SPZ) has become the framework for the elucidation of protective immune mechanisms and the prototype for a promising vaccine strategy. We have previously reported that although considered stage specific based on antibody and CD8+ cytolytic T lymphocyte responses directed against preerythrocytic stage antigens, in particular, the circumsporozoite protein and sporozoite surface protein 2, protective immunity induced in humans by attenuated Plasmodium falciparum SPZ may also involve CD4+ T cell responding to antigens present on parasitized red blood cells (pRBC). In this study we examined the functional role of pRBC responding CD4+ T cells by comparing in vitro pRBC-stimulated responses of CD4+ T cells from persons during preimmunity to irradiated SPZ, during induction of protection, and infection induced with SPZ. The results reported herein corroborate previously published observations that antigens associated with pRBC induce proliferative CD4+ lymphocytes responses in subjects exposed to malaria parasite-derived antigens and not malaria-naive persons; however, now we demonstrate that pRBC-proliferative CD4+ T cells did not coincide with protective immunity. Similarly, pRBC-induced IFN-gamma levels did not distinguish malaria protected from susceptible persons, although IFN-gamma was observed only in lymphocyte cultures from malaria parasite-exposed volunteers and not in lymphocyte cultures from malaria-naive persons. In contrast, we noted an increase in the IL-4-producing CD4+ T cells that also exhibited the memory phenotype, CD45RO, and an upregulated expression of CD25 in cultures from malaria protected persons as compared to malaria naive persons and subjects who became parasitemic. Hence, these observations suggest that the induction of memory CD4+ T cell subset distinguished by the expression of CD45RO and CD25 and production of IL-4 coincides with protective immune responses generated by immunization with attenuated SPZ.

Plasmodium falciparum malaria blood stage parasites preferentially inhibit macrophages with high phagocytic activity

Exposure to malaria blood stage antigens results in several defects of macrophages/monocytes one of which is an irreversible reduction of phagocytic activity. In the present study we analysed phagocytic activity of subpopulations of human monocyte-derived-macrophages (MDM) based on the capacity of individual cells to ingest FITC-labelled microbeads. The results demonstrate that malaria infection affected predominantly MDM subpopulations with high level of phagocytosis. This population decreased during parasitaemia, however, during recovery from the infection the highly phagocytic cells replaced the damaged cells. The exposure of MDM cultures to blood stage antigens showed that the highly active macrophages from persons with active malaria infection decreased further, while the population increased during recovery. Furthermore, we observed that while ingestion of a few parasitized RBC (3 schizonts) stimulated phagocytosis, larger amounts or longer exposure periods eventually paralysed the entire phagocytic system. Accordingly, by selectively blocking actively phagocytizing macrophages, the malaria parasite prevents both specific and non-specific immune responses, which are initiated by macrophages as phagocytes and professional antigen presenting cells.

A preliminary evaluation of a recombinant circumsporozoite protein vaccine against Plasmodium falciparum malaria. RTS,S Malaria Vaccine Evaluation Group

BACKGROUND

The candidate vaccines against malaria are poorly immunogenic and thus have been ineffective in preventing infection. We developed a vaccine based on the circumsporozoite protein of Plasmodium falciparum that incorporates adjuvants selected to enhance the immune response.

METHODS

The antigen consists of a hybrid in which the circumsporozoite protein fused to hepatitis B surface antigen (HBsAg) is expressed together with unfused HBsAg. We evaluated three formulations of this antigen in an unblinded trial in 46 subjects who had never been exposed to malaria.

RESULTS

Two of the vaccine formulations were highly immunogenic. Four subjects had adverse systemic reactions that may have resulted from the intensity of the immune response after the second dose, which led us to reduce the third dose. Twenty-two vaccinated subjects and six unimmunized controls underwent a challenge consisting of bites from mosquitoes infected with P. falciparum. Malaria developed in all six control subjects, seven of eight subjects who received vaccine 1, and five of seven subjects who received vaccine 2. In contrast, only one of seven subjects who received vaccine 3 became infected (relative risk of infection, 0.14; 95 percent confidence interval, 0.02 to 0.88; P<0.005).

CONCLUSIONS

A recombinant vaccine based on fusion of the circumsporozoite protein and HBsAg plus a potent adjuvant can protect against experimental challenge with P. falciparum sporozoites. After additional studies of protective immunity and the vaccination schedule, field trials are indicated for this new vaccine against P. falciparum malaria.

Safety, immunogenicity, and efficacy of Plasmodium falciparum repeatless circumsporozoite protein vaccine encapsulated in liposomes

Seventeen malaria-naive volunteers received a recombinant Plasmodium falciparum vaccine (RLF) containing the carboxy- and the amino-terminal of the circumsporozoite protein (CSP) antigen without the central tetrapeptide repeats. The vaccine was formulated in liposomes with either a low or high dose of 3-deacylated monophosphoryl lipid A (MPL) and administered with alum by intramuscular injection. Both formulations were well tolerated and immunogenic. MPL increased sporozoite antibody titers measured by ELISA, Western blot, and immunofluorescence assay. One high-dose MPL vaccine formulation recipient developed a CSP-specific cytotoxic T lymphocyte response. After homologous sporozoite challenge, immunized volunteers developed patent malaria. There was no correlation between prepatent period and antibody titers to the amino- or carboxy-terminal. The absence of delay in patency argues against inclusion of the amino-terminal in future vaccines. A significant cytotoxic T lymphocyte response may have been suppressed by the inclusion of alum as an adjuvant.

MHC class I-dependent presentation of exoerythrocytic antigens to CD8+ T lymphocytes is required for protective immunity against Plasmodium berghei

T lymphocytes are believed to play a major role in protection against malaria. Previous experiments using in vivo depletion of CD8+ T cells, reconstitution with CD8+ T splenic cells, and adoptive transfer of CD8+ CTL clones demonstrated that protection against the exoerythrocytic stage of the murine strain, Plasmodium berghei malaria, was CD8+ T cell-dependent. Despite evidence for the critical role of CD8+ CTL, neither the cellular nor the molecular requirements for CD8+ T cell induction or for recognition of malaria Ags are known. In this study, we wished to define the role of CD8+ T cells and MHC class I molecules by using the P. berghei malaria attenuated sporozoites (SPZ) protection model in beta 2-microglobulin (beta 2m) knockout (-/-) mice. In contrast to observations that beta 2m-/- mice are resistant to many infectious diseases by compensatory mechanisms involving non-class I-restricted T cells, we found that beta 2m-/- mice failed to be protected against P. berghei SPZ, although immunization with attenuated SPZ induced production of IL-2, INF-gamma, anti-circumsporozoite protein IgG, and proliferative T cells. The lack of compensatory mechanisms involving non-CD8+ T cells was particularly evident in the failure to adoptively transfer protective immunity with wild-type SPZ-immune splenic T cells. From our data it can be concluded that CD8+ T cells induced during immunization with attenuated SPZ must recognize liver-expressed Ags presented by class I molecules to engage effectively in a response leading to destruction of the malaria parasites.

MHC class I-dependent presentation of exoerythrocytic antigens to CD8+ T lymphocytes is required for protective immunity against Plasmodium berghei

T lymphocytes are believed to play a major role in protection against malaria. Previous experiments using in vivo depletion of CD8+ T cells, reconstitution with CD8+ T splenic cells, and adoptive transfer of CD8+ CTL clones demonstrated that protection against the exoerythrocytic stage of the murine strain, Plasmodium berghei malaria, was CD8+ T cell-dependent. Despite evidence for the critical role of CD8+ CTL, neither the cellular nor the molecular requirements for CD8+ T cell induction or for recognition of malaria Ags are known. In this study, we wished to define the role of CD8+ T cells and MHC class I molecules by using the P. berghei malaria attenuated sporozoites (SPZ) protection model in beta 2-microglobulin (beta 2m) knockout (-/-) mice. In contrast to observations that beta 2m-/- mice are resistant to many infectious diseases by compensatory mechanisms involving non-class I-restricted T cells, we found that beta 2m-/- mice failed to be protected against P. berghei SPZ, although immunization with attenuated SPZ induced production of IL-2, INF-gamma, anti-circumsporozoite protein IgG, and proliferative T cells. The lack of compensatory mechanisms involving non-CD8+ T cells was particularly evident in the failure to adoptively transfer protective immunity with wild-type SPZ-immune splenic T cells. From our data it can be concluded that CD8+ T cells induced during immunization with attenuated SPZ must recognize liver-expressed Ags presented by class I molecules to engage effectively in a response leading to destruction of the malaria parasites.

T lymphocytes from volunteers immunized with irradiated Plasmodium falciparum sporozoites recognize liver and blood stage malaria antigens

The model of protective immunity induced by immunization with irradiated plasmodia sporozoites (SPZ) has become the prototype for a promising vaccine strategy based on Ab and CTL responses directed against pre-erythrocytic stage Ags, in particular the circumsporozoite protein (CSP) and sporozoite surface protein 2 (SSP2). However, results from recently conducted vaccine studies suggest that T cell responses directed against additional specificities might also be required for protection. We have tested this hypothesis by examining human T lymphocytes from irradiated Plasmodium falciparum SPZ-immune volunteers for proliferative reactivities to parasitized red blood cells (pRBC) and recombinant proteins and synthetic peptides representing certain liver and blood stage Ags. In this work, we report that although SPZ-induced protective immunity is stage-specific, SPZ-immune lymphocytes recognized determinants associated with erythrocytic and liver stage parasites. Thus, protective immunity induced by irradiated SPZ may depend upon responses against pre-erythrocytic Ags in addition to CSP and SSP2.

T lymphocytes from volunteers immunized with irradiated Plasmodium falciparum sporozoites recognize liver and blood stage malaria antigens

The model of protective immunity induced by immunization with irradiated plasmodia sporozoites (SPZ) has become the prototype for a promising vaccine strategy based on Ab and CTL responses directed against pre-erythrocytic stage Ags, in particular the circumsporozoite protein (CSP) and sporozoite surface protein 2 (SSP2). However, results from recently conducted vaccine studies suggest that T cell responses directed against additional specificities might also be required for protection. We have tested this hypothesis by examining human T lymphocytes from irradiated Plasmodium falciparum SPZ-immune volunteers for proliferative reactivities to parasitized red blood cells (pRBC) and recombinant proteins and synthetic peptides representing certain liver and blood stage Ags. In this work, we report that although SPZ-induced protective immunity is stage-specific, SPZ-immune lymphocytes recognized determinants associated with erythrocytic and liver stage parasites. Thus, protective immunity induced by irradiated SPZ may depend upon responses against pre-erythrocytic Ags in addition to CSP and SSP2.

Intracellular processing of liposome-encapsulated antigens by macrophages depends upon the antigen

Two proteins, a recombinant malaria protein (R32NS1) and conalbumin, were encapsulated in separate liposomes. The mechanisms of presentation of unencapsulated and liposome-encapsulated R32NS1 and conalbumin to antigen-specific T-cell clones were investigated in in vitro antigen presentation assays using murine bone marrow-derived macrophages (BMs) as antigen-presenting cells. A much lower concentration of liposomal antigen than of unencapsulated antigen was required for T-cell proliferation. Liposome-encapsulated conalbumin required intracellular processing by BMs for antigen-specific T-cell proliferation, as determined by inhibition with chloroquine, NH4Cl, leupeptin, brefeldin A, monensin, antimycin A, NaF, and cycloheximide and by treatment of BMs with glutaraldehyde. Liposome-encapsulated conalbumin therefore follows the classical intracellular antigen processing pathway described for protein antigens. Similarly, unencapsulated conalbumin also required intracellular processing for presentation to antigen-specific T cells. In contrast, both unencapsulated R32NS1 and liposome-encapsulated R32NS1 were presented to T cells by BMs without undergoing internalization and intracellular processing. These results suggest that the antigen itself is the major element that determines whether a requirement exists for intracellular processing of liposomal antigens by macrophages.

Safety, immunogenicity, and efficacy of a recombinantly produced Plasmodium falciparum circumsporozoite protein-hepatitis B surface antigen subunit vaccine

Twenty malaria-naive volunteers received a recombinant Plasmodium falciparum malaria vaccine (RTS,S) containing 19 NANP repeats and the carboxy terminus (amino acids 210-398) of the circumsporozoite (CS) antigen coexpressed in yeast with hepatitis B surface antigen. Ten received vaccine adjuvanted with alum, and 10 received vaccine adjuvanted with alum plus 3-deacylated monophosphoryl lipid A (MPL). Both formulations were well tolerated and immunogenic. MPL enhanced CS antibody levels (measured by ELISA, immunofluorescence, and inhibition of sporozoite invasion assays). After sporozoite challenge, 6 of 6 in the alum group and 6 of 8 in the alum-MPL group developed patent malaria. Protected subjects had higher levels of CS antibody titers on day of challenge than did nonprotected subjects. After immunization, 1 protected subject had increased cytotoxic T lymphocyte activity against CS and recall of memory T cell responses to RTS,S and selected CS.

Immunization with irradiated Plasmodium berghei sporozoites induces IL-2 and IFN gamma but not IL-4

Protective immunity against Plasmodium induced by immunization with irradiated sporozoites (SPZ) depends on both humoral and cellular responses. Although circumsporozoite protein (CSP)-specific cytolytic T lymphocyte responses have been established as an effector system, other cell types are required for protection. We have previously demonstrated that although protective immunity and T cell proliferative reactivity to SPZ are mouse strain- and SPZ dose-dependent, no correlation between the two responses could be found. Since protective immunity involves functionally diverse T cell subsets, we asked whether the discordance between proliferative responses to SPZ and protective immunity might have resulted from selective activation of either the Th1 or Th2 cell subset. Protective immunity, in vitro proliferative responses, and lymphokine production were tested in BALB/c, C57Bl/6, and C3H/HeN mice immunized according to different SPZ regimens. The levels of IL-2 paralleled the proliferative reactivities in each mouse strain examined. Although IFN gamma levels were present in the unprimed lymphocyte cultures, they increased following each SPZ immunization, in C57Bl/6, moderate in C3H/HeN, and lowest in BALB/c splenic cultures. Surprisingly, no IL-4 was detected in splenic cultures from any mouse strain during proliferative activity or protective immunity. In contrast, elevated IL-6 production was noted after each immunization, regardless of the protective status and it correlated with anti-CSP IgG serum levels. These data establish that lymphokine profiles corresponding primarily to the Th1 cells were induced by immunization with P. berghei SPZ and that IL-4 secreting T cells were not induced by the SPZ-stage berghei antigens.

Induction of cytolytic and antibody responses using Plasmodium falciparum repeatless circumsporozoite protein encapsulated in liposomes

Plasmodium circumsporozoite (CS) protein-induced antibody and T-cell responses are considered to be important in protective immunity. Since the key repeat determinant of the CS protein may actually restrict the recognition of other potential T- and B-cell sites, a modified Plasmodium falciparum CS protein lacking the central repeat region, RLF, was expressed in Escherichia coli. On purification, RLF was encapsulated into liposomes [L(RLF)] and used for the in vivo induction of cytolytic T lymphocytes (CTL) and antibodies. Immunization of B10.Br (H-2k) mice with L(RLF), but not with RLF, induced CD8+ CTL specific for the P. falciparum CS protein CTL epitope, amino acid residues 368-390. Anti-L(RLF) serum reacted with antigens on intact sporozoites and inhibited sporozoite invasion of hepatoma cells. Antibody specificity studies in New Zealand White rabbits revealed new B-cell sites localized in amino acid residues 84-94, 91-99, 97-106 and 367-375. Although the mechanisms by which liposomes enhance cellular and humoral immune responses remain unknown, liposome-formulated vaccines have been well tolerated in humans; hence, their use in vaccines, when efficacy depends on antibody and CTL responses, may be broadly applicable.

Plasmodium berghei-specific T cells respond to non-processed sporozoites presented by B cells

The mechanism of malaria protective immunity induced by immunization with radiation-attenuated Plasmodium sporozoites (SPZ) is only partially understood. For example, B and T cell responses specific for the circumsporozoite (CS) protein, a 46 kDa SPZ surface protein, have been characterized; however, events leading to SPZ-specific T cell activation, i.e., processing and presentation of SPZ by antigen-presenting cells have not been investigated. In the present study we describe the in vitro analysis of requirements for accessory cell function in the presentation of SPZ to SPZ-immune T cells. The results establish that SPZ-induced proliferative T cells are reactive to non-processed SPZ presented by activated B cells and, thus, imply that the non-processed form of the SPZ-associated CS protein restricts the induction of the potential CS protein T cell repertoire.

Adjuvant effects of liposomes containing lipid A: enhancement of liposomal antigen presentation and recruitment of macrophages

Liposomes containing lipid A induced potent humoral immune responses in mice against an encapsulated malaria antigen (R32NS1) containing NANP epitopes. The immune response was not enhanced by lipid A alone or by empty liposomes containing lipid A. Experiments to investigate the adjuvant mechanisms of liposomes and lipid A revealed that liposome-encapsulated R32NS1 was actively presented by bone marrow-derived macrophages to NANP-specific cloned T cells. The degree of presentation was related to the amount of liposomal antigen added per macrophage in the culture medium. At high cell densities, poor presentation occurred when liposomes lacked lipid A but excellent presentation occurred when the liposomes contained lipid A. Liposomes containing lipid A and encapsulated antigen also activated gamma interferon-treated macrophages to produce nitric oxide. Macrophage activation and antigen presentation occurred with liposomes that could not be detected by the Limulus amebocyte lysis assay. Intraperitoneal injection of liposomal lipid A caused a marked increase in the recruitment of immature (peroxidase-positive) macrophages to the peritoneum. On the basis of these experiments, we propose that the mechanism of the adjuvant action of liposomal lipid A is partly due to increased antigen presentation by macrophages and partly due to recruitment of an increased number of macrophages serving as antigen-presenting cells.

Distinct T cell specificities are induced with the authentic versus recombinant Plasmodium berghei circumsporozoite protein

Plasmodium berghei sporozoite (SPZ)-immune lymph node (LN) cells obtained from mice of different H-2 haplotypes were analyzed for the presence of circumsporozoite (CS) protein-reactive T cells in proliferative assays. Although lymphocytes from each strain responded in vitro to the priming Ag and to the soluble rCS protein, they did not respond to CS protein synthetic peptides. Parallel analysis of rCS protein-primed LN cells revealed that the two Ag are unequal in generating T cell specificities: although SPZ priming did not induce CS protein peptide-reactive T cells, priming with rCS protein did. Not being privy to the processing and presentation of SPZ Ag, we postulated that a different order of processing of the authentic, i.e., SPZ-associated CS protein vs soluble rCS protein might be responsible for the generation of different T cell specificities. Accordingly, authentic CS protein might not be processed by APC, or the processed fragments might obscure the recognition of smaller peptide fragments. Therefore, we subjected the SPZ to three cycles of a freeze/thaw procedure and used the denatured SPZ preparation for priming. We observed that contrary to priming with the authentic SPZ, denatured SPZ generated T cells reactive to some of the CS protein synthetic peptides. The hypothesis that each form of the SPZ Ag is subject to a unique Ag processing was also confirmed in experiments demonstrating a lack of recognition of the authentic CS protein by rCS protein-primed LN cells. Hence, the evidence presented in this work that complex protozoan Ag, such as Plasmodia, might present different requirements for Ag-specific T cell induction/activation not only enhances the basic understanding of the immune system, but is essential for the development of antimalaria vaccine(s). In addition, these observations support the hypothesis that the molecular context of the priming Ag influences the outcome of T cell specificities, by providing evidence that the authentic CS protein induces a T cell repertoire that is distinct from that induced by the rCS protein.

Distinct T cell specificities are induced with the authentic versus recombinant Plasmodium berghei circumsporozoite protein

Plasmodium berghei sporozoite (SPZ)-immune lymph node (LN) cells obtained from mice of different H-2 haplotypes were analyzed for the presence of circumsporozoite (CS) protein-reactive T cells in proliferative assays. Although lymphocytes from each strain responded in vitro to the priming Ag and to the soluble rCS protein, they did not respond to CS protein synthetic peptides. Parallel analysis of rCS protein-primed LN cells revealed that the two Ag are unequal in generating T cell specificities: although SPZ priming did not induce CS protein peptide-reactive T cells, priming with rCS protein did. Not being privy to the processing and presentation of SPZ Ag, we postulated that a different order of processing of the authentic, i.e., SPZ-associated CS protein vs soluble rCS protein might be responsible for the generation of different T cell specificities. Accordingly, authentic CS protein might not be processed by APC, or the processed fragments might obscure the recognition of smaller peptide fragments. Therefore, we subjected the SPZ to three cycles of a freeze/thaw procedure and used the denatured SPZ preparation for priming. We observed that contrary to priming with the authentic SPZ, denatured SPZ generated T cells reactive to some of the CS protein synthetic peptides. The hypothesis that each form of the SPZ Ag is subject to a unique Ag processing was also confirmed in experiments demonstrating a lack of recognition of the authentic CS protein by rCS protein-primed LN cells. Hence, the evidence presented in this work that complex protozoan Ag, such as Plasmodia, might present different requirements for Ag-specific T cell induction/activation not only enhances the basic understanding of the immune system, but is essential for the development of antimalaria vaccine(s). In addition, these observations support the hypothesis that the molecular context of the priming Ag influences the outcome of T cell specificities, by providing evidence that the authentic CS protein induces a T cell repertoire that is distinct from that induced by the rCS protein.

Use of adjuvant containing mycobacterial cell-wall skeleton, monophosphoryl lipid A, and squalane in malaria circumsporozoite protein vaccine

Human immune responses to modern synthetic and recombinant peptide vaccines administered with the standard adjuvant, aluminum hydroxide, tend to be poor, hence the search for better adjuvants. Antibody responses to a Plasmodium falciparum circumsporozoite (CS) protein vaccine, R32NS1(81), administered with an adjuvant containing cell-wall skeleton of mycobacteria and monophosphoryl lipid A in squalane (MPL/CWS) have been compared to responses to the same immunogen administered with aluminum hydroxide. 2 weeks after the third dose the following indices were greater in the 5 patients who received MPL/CWS than in controls (p less than 0.05): the geometric mean concentration (2.0 vs 25.4 microgram/ml) and avidity index of antibodies to the P falciparum CS protein by ELISA, the geometric mean titre to P falciparum sporozoites by IFAT (1/115 vs 1/1600), and the geometric mean inhibition of sporozoite invasion of hepatoma cells in vitro (37.6 vs 90.3%). For R32NS1(81) MPL/CWS is superior to aluminum hydroxide as an adjuvant, and the data support the evaluation of this complex as an adjuvant for other vaccines.

The distinctive specificity of antigen-specific suppressor T cells

Although suppressor T cells have been cloned in only a few instances, the existence of a functional cadre of T cells that acts to downregulate the immune response is well documented. In this review Eli Sercarz and Urszula Krzych describe studies on suppressor T-cell (TS-cell) specificity that provide some support for the conclusion that the TS cell is a distinctive cell type with an expressed repertoire that is different from that expressed by helper T (TH) cells. They go on to explore the interaction between cells recognizing TS-cell-inducing determinants (SDs) and TH-cell-inducing determinants (HDs), and their relationship to immunogenicity and Ir gene effects.

Immune response to Plasmodium berghei sporozoite antigens. I. Evaluation of murine T cell repertoire following immunization with irradiated sporozoites

The Plasmodium berghei sporozoite antigen-specific T cell repertoire was analyzed in C57BL/6 (H-2b), BALB/c (H-2d) and C3H/HeN (H-2k) mice following immunization with irradiated sporozoites. Proliferative responses were correlated with the protective status of each strain. Proliferative reactivities to sporozoite antigens were compared in cultures containing either CD4+ T cells, CD8+ T cells, or total splenic lymphocytes. CD8+ T cells had no proliferative activity to sporozoite antigens; CD4+ T cells and splenic lymphocytes responded to the priming antigen, but the responses varied according to the mouse strain tested. The proliferative activity diminished at the onset of protection, presumably due to the induction of regulatory or non-proliferative T cell subsets. Sporozoite-immune lymphocytes did not respond to P. berghei circumsporozoite synthetic peptides. The restricted utilization of T cell epitopes during anti-sporozoite responses can be interpreted as resulting in part from a limited processing of the CS protein antigen.

Role of circumsporozoite protein-specific T-cells in protective immunity against Plasmodium berghei

The Plasmodium berghei circumsporozoite (CS) protein-specific T-cell repertoire was analysed in C57Bl/6 (H-2b), Balb/c (H-2d), and C3H/HeN (H-2k) mice immunized with irradiated sporozoites and the proliferative responses were correlated with the protective status of each strain. Splenic lymphocytes responded to the priming antigen, but the responses varied according to both murine strain and immunization schedule. Analysis of cytolytic T lymphocyte (CTL) responses in mice immunized with irradiated P. berghei sporozoites or with a Salmonella-recombinant CS protein construct revealed that each immunization induces CTLs recognizing different epitopes on the target cells. The variations in immune reactivities among different murine strains to the CS protein antigens and the variations in the responses to the authentic versus recombinantly expressed CS protein suggest that distinct immune mechanisms may be involved in rendering immune protection. Furthermore, the molecular context of the immunizing antigen may influence the outcome of the fine specificity of T cells involved in immune protection.

Sequences outside a minimal immunodominant site exert negative effects on recognition by staphylococcal nuclease-specific T cell clones

In recent years, synthetic peptides have been utilized extensively to characterize the minimal essential immunodominant sites on model protein Ag. However, little work has focused on the effect that sequences flanking these minimal recognition sites may exert on T cell recognition. Previous work with staphylococcal nuclease (Nase) demonstrated that I-Ek-restricted clones recognize the peptide 81-100, whereas I-Ab-restricted clones recognize the over-lapping but non-cross-reacting peptide 91-110. Further analysis with 15 or 10 residue peptides within the region 81-110 reveals that the minimal sequence capable of stimulating I-Ek-restricted clones is contained within the decapeptide 91-100. Addition of residues 86-90, to give the peptide 86-100, enhanced the recognition substantially, whereas addition of residues 101-105 produced a 91-105 peptide with no stimulatory ability. These results suggest that interactions between the antigenic peptide 91-100 and residues within the flanking 101-105 sequence have negative consequences for presentation of the immunodominant epitope to T cell clones. Introduction of single amino acid substitutions within 91-105 produced peptides that induce responses comparable to those seen with 91-100. These results are consistent with the suggestion of negative interactions between the minimal immunodominant site and flanking sequences in that single residue substitutions may remove these negative interactions and lead to restoration of stimulatory ability. The negative effect of flanking sequences on T cell recognition of immunodominant sites presents new considerations for development of synthetic vaccines as well as for understanding the biology of Ag processing and presentation.

Sequences outside a minimal immunodominant site exert negative effects on recognition by staphylococcal nuclease-specific T cell clones

In recent years, synthetic peptides have been utilized extensively to characterize the minimal essential immunodominant sites on model protein Ag. However, little work has focused on the effect that sequences flanking these minimal recognition sites may exert on T cell recognition. Previous work with staphylococcal nuclease (Nase) demonstrated that I-Ek-restricted clones recognize the peptide 81-100, whereas I-Ab-restricted clones recognize the over-lapping but non-cross-reacting peptide 91-110. Further analysis with 15 or 10 residue peptides within the region 81-110 reveals that the minimal sequence capable of stimulating I-Ek-restricted clones is contained within the decapeptide 91-100. Addition of residues 86-90, to give the peptide 86-100, enhanced the recognition substantially, whereas addition of residues 101-105 produced a 91-105 peptide with no stimulatory ability. These results suggest that interactions between the antigenic peptide 91-100 and residues within the flanking 101-105 sequence have negative consequences for presentation of the immunodominant epitope to T cell clones. Introduction of single amino acid substitutions within 91-105 produced peptides that induce responses comparable to those seen with 91-100. These results are consistent with the suggestion of negative interactions between the minimal immunodominant site and flanking sequences in that single residue substitutions may remove these negative interactions and lead to restoration of stimulatory ability. The negative effect of flanking sequences on T cell recognition of immunodominant sites presents new considerations for development of synthetic vaccines as well as for understanding the biology of Ag processing and presentation.

The molecular context of determinants within the priming antigen establishes a hierarchy of T cell induction: T cell specificities induced by peptides of beta-galactosidase vs. the whole antigen

The antibody response following priming with a macromolecule or a peptide will depend on the regulatory T cells that become activated by the antigenic determinants available. In this report, activation of T helper (Th) and T suppressor (Ts) cells by determinants on beta-galactosidase (GZ) was examined by comparing native GZ [1023 amino acid (a.a.) residues per monomer] with peptides from the immunodominant region encompassing residues 3 to 187. Each immunogen established its characteristic hierarchy of dominance of determinants within it: in particular, GZ and CB-2-3 (a.a. 3-187) each induced immunodominant Th cells which could not be induced by T8 (a.a. 60-140). Hierarchies of suppressor determinant are also created: T8-Ts suppresses all Th specificities and therefore can be deemed immunodominant: T8-2-Ts and T8-3-Ts have a more selective suppressor activity and can be considered subdominant. We conclude that the outcome of immunization shifts with a change in the nature of the immunogen and the context within which the determinant lies will crucially influence its expression. A particular "context" presumably determines the likely order of processing of that molecule which leads to a characteristic relationship among the Ts, Th and B cell determinants involved.

Induction of helper and suppressor T cells by nonoverlapping determinants on the large protein antigen, beta-galactosidase

The fine specificity of the T cell repertoire directed against T helper (Th)-inducing and T suppressor (Ts)-inducing determinants was examined with cyanogen bromide and tryptic peptides of Escherichia coli beta-galactosidase (GZ), a large tetrameric protein (monomer molecular weight = 116 kDa). Immunization with cyanogen bromide fragment 2 [CB-2, amino acids (a.a.) 3-92] induced both specific Th and Ts cells. Study of the induction of these functionally opposite T cell subpopulations with tryptic peptides of CB-2 indicated that Th and Ts were activated by separate, nonoverlapping determinants. Th-inducing activity resided in a nonapeptide, T6 (a.a. 44-52), whereas T4 (a.a. 27-37) induced Ts cells. The presence of distinct helper and suppressor determinants suggests that the specificity repertoire in these T cell subpopulations may differ, perhaps owing to the expression of antigen-recognizing receptors that are coded by unique gene families. Alternatively, antigen presentation structures may be physicochemically quite different, and bind to distinct parts of the peptide antigen.

Repertoires of T cells directed against a large protein antigen, beta-galactosidase. II. Only certain T helper or T suppressor cells are relevant in particular regulatory interactions

11 cyanogen bromide (CB) peptides, comprising 70% of the large protein, Escherichia coli beta-galactosidase (GZ), were studied for their ability to induce T suppressor (Ts) cells capable of strongly suppressing the in vitro anti-fluorescein (FITC) response to GZ-FITC. Only CB-2 (amino acid residues 3-92) and CB-3 (residues 93-187) were found to bear such Ts-inducing epitopes. In examining the specificity of T helper cell (Th) targets susceptible to CB-2 and CB-3-specific Ts, it appeared that only nearly Th targets could be suppressed. Thus, CB-10-primed Th were not suppressed by either Ts; even CB-3-primed Ts did not suppress CB-2-specific Th, although CB-2-specific Ts were effective. Furthermore, analysis of the suppression pattern revealed a hierarchical use of potential epitopes on native GZ in triggering functional regulatory T cells. A dominant Th epitope near the amino terminus of GZ tops a hierarchy of potential Th, most of which are never engaged. The dominant determinant seems to exist on the peptide CB-2-3 (residues 3-187), and presumably is destroyed by its cleavage at Met 92; the Th cells that it induces are suppressible by each of the Ts-inducing peptides. In the GZ system, where the native antigen is quite large, the interactions between Th and Ts are highly circumscribed. This may be attributable to the topology of antigen fragments produced during processing; any relevant fragment must bear at least a Ts- and Th-reactive determinant to permit intercellular regulation. A final implication of these results is that, not only does the existence of a Th-inducing determinant depend on its being an appropriate distance from a B cell epitope, but the existence of Ts-inducing determinants likewise depends on the existence of a neighboring Th-B cell association.

Repertoires of T cells directed against a large protein antigen, beta-galactosidase. I. Helper cells have a more restricted specificity repertoire than proliferative cells

The proliferative and helper T cell repertoires were compared in the CBA/J mouse for the response to the large protein antigen, tetrameric beta-galactosidase (GZ = 1021 a.a/monomer). The systems assessed the ability of cyanogen bromide (CB) peptides of GZ to: 1) prime for a T cell proliferative response to GZ; or 2) generate T cell help, measured by the production of anti-FITC PFC in the in vitro response to GZ-FITC. Priming for in vitro proliferation was attempted with 11 CB peptides comprising 70% of the GZ molecule. Strong priming was found with five peptides and intermediate priming was found with four other peptides; two peptides were without effect (CB-20 = a.a 767-862, and CB-4 = a.a. 188-202). Despite this indication of generally dispersed recognition of GZ epitopes, only two CB peptides, CB-2 (a.a. 3-92) and CB-10 (a.a. 378-418) were able to induce a T helper cell response. The surprising dearth of helper T cell-inducing epitopes may be peculiar to the limited fluorescein (FITC) substitution on GZ-FITC (17-25 FITC residues per tetramer) or it may reflect the constraints involved in T cell recognition required for T-B collaboration. Also considered was the possibility that the helper T cell repertoire might be distinct from the proliferative repertoire, the latter reflecting DNA synthesis and recruitment by other functional T cell subpopulations.

Repertoires of T cells directed against a large protein antigen, beta-galactosidase. I. Helper cells have a more restricted specificity repertoire than proliferative cells

The proliferative and helper T cell repertoires were compared in the CBA/J mouse for the response to the large protein antigen, tetrameric beta-galactosidase (GZ = 1021 a.a/monomer). The systems assessed the ability of cyanogen bromide (CB) peptides of GZ to: 1) prime for a T cell proliferative response to GZ; or 2) generate T cell help, measured by the production of anti-FITC PFC in the in vitro response to GZ-FITC. Priming for in vitro proliferation was attempted with 11 CB peptides comprising 70% of the GZ molecule. Strong priming was found with five peptides and intermediate priming was found with four other peptides; two peptides were without effect (CB-20 = a.a 767-862, and CB-4 = a.a. 188-202). Despite this indication of generally dispersed recognition of GZ epitopes, only two CB peptides, CB-2 (a.a. 3-92) and CB-10 (a.a. 378-418) were able to induce a T helper cell response. The surprising dearth of helper T cell-inducing epitopes may be peculiar to the limited fluorescein (FITC) substitution on GZ-FITC (17-25 FITC residues per tetramer) or it may reflect the constraints involved in T cell recognition required for T-B collaboration. Also considered was the possibility that the helper T cell repertoire might be distinct from the proliferative repertoire, the latter reflecting DNA synthesis and recruitment by other functional T cell subpopulations.

Effects of sex hormones on some T and B cell functions, evidenced by differential immune expression between male and female mice and cyclic pattern of immune responsiveness during the estrous cycle in female mice

The responses of spleen cells from male and female BALB/c mice were evaluated to determine if sex-related variations in immune expression could be found. The immunologic assays used included blastogenic responses to mitogens, and direct and indirect measurement of plaque-forming cells against particulate antigens. The results indicated that responses of spleen cells from young adult female mice were higher than those of males in all comparative tests. Newborn mice did not demonstrate the sex-associated immune differences; and among the weanling mice slight differences between male and female spleen cells responsiveness to mitogenic agents were observed. The blastogenic responsiveness of spleens from female BALB/c was greater at proestrus and metestrus, as compared to estrus and diestrus. The peaks of responsiveness corresponded to reported elevated levels of estrogen and pregnenolone during these phases of the cycle. Similar results were obtained with the IgM plaque-forming cell responses, which were also increased at proestrus and metestrus. This study supports a role of sex hormones in modulation of immune expression.

Sex-related immunocompetence of BALB/c mice. I. Study of immunologic responsiveness of neonatal, weanling, and young adult mice

The responses of lymphoid cells from the thymus, lymph nodes, and spleen of male and female BALB/c mice were evaluated to determine if sex-related variations in immune expression could be found. Immunologic assays used included blastogenic responses to mitogens, mixed lymphocyte responses, and direct and indirect measurement of plaque-forming cells against soluble and particulate antigens. The results indicated that responses of spleen cells from young adult female mice were higher than those of males in all comparative tests. Little or no differences between the sexes were observed in the mitogenesis of lymph nodes and thymuses. Newborn mice did not demonstrate the sex-associated immune differences. Among the weanling mice slight differences between male and female spleen cells responsiveness to mitogenic agents were observed.

Quantitative differences in immune responses during the various stages of the estrous cycle in female BALB/c mice

The immune responsiveness of spleens from female BALB/c mice to PHA, Con A, and LPS was greater at proestrus and metestrus as compared with estrus and diestrus. The peaks of responsiveness corresponded to reported elevated levels of estrogen and pregnenolone during these phases of the cycle. Similar results were obtained with the IgM or direct plaque-forming cell responses, which were also increased at proestrus and metestrus. It appears that female hormones may directly or indirectly stimulate immune responsiveness in adult mice.