Major histocompatibility complex (mhc)-encoded molecules govern immune responses by presenting antigenic peptides to T cells. The extensive polymorphism of genes encoding these molecules is believed to enhance immune defense by broadening the array of antigenic peptides available for T cell recognition, but direct evidence supporting the importance of this mechanism in combating pathogens is limited. Here we link mhc polymorphism-driven diversification of the cytotoxic T lymphocyte (CTL) repertoire to the generation of high-avidity, protective antiviral T cells and to superior antiviral defense. Thus, much of the beneficial effect of the mhc polymorphism in immune defense may be due to its critical influence on the properties of the selected CTL repertoire.

Merozoite surface protein–1 (MSP-1) of the human malaria parasite Plasmodium falciparum undergoes at least two endoproteolytic cleavage events during merozoite maturation and release, and erythrocyte invasion. We have previously demonstrated that mAbs which inhibit erythrocyte invasion and are specific for epitopes within a membrane-proximal, COOH-terminal domain of MSP-1 (MSP-119) prevent the critical secondary processing step which occurs on the surface of the extracellular merozoite at around the time of erythrocyte invasion. Certain other anti–MSP-119 mAbs, which themselves inhibit neither erythrocyte invasion nor MSP-1 secondary processing, block the processing-inhibitory activity of the first group of antibodies and are termed blocking antibodies. We have now directly quantitated antibody-mediated inhibition of MSP-1 secondary processing and invasion, and the effects on this of blocking antibodies. We show that blocking antibodies function by competing with the binding of processing-inhibitory antibodies to their epitopes on the merozoite. Polyclonal rabbit antibodies specific for certain MSP-1 sequences outside of MSP-119 also act as blocking antibodies. Most significantly, affinity-purified, naturally acquired human antibodies specific for epitopes within the NH2-terminal 83-kD domain of MSP-1 very effectively block the processing-inhibitory activity of the anti-MSP-119 mAb 12.8. The presence of these blocking antibodies also completely abrogates the inhibitory effect of mAb 12.8 on erythrocyte invasion by the parasite in vitro. Blocking antibodies therefore (a) are part of the human response to malarial infection; (b) can be induced by MSP-1 structures unrelated to the MSP-119 target of processing-inhibitory antibodies; and (c) have the potential to abolish protection mediated by anti–MSP-119 antibodies. Our results suggest that an effective MSP-119–based falciparum malaria vaccine should aim to induce an antibody response that prevents MSP-1 processing on the merozoite surface.

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The immunogenicity and protective efficacy of four versions of recombinant C-terminal 19-kDa epidermal growth factor-like region of the major surface protein 1 (rMSP119) of Plasmodium falciparum was studied in Aotus monkeys. Vaccination with each of the four rMSP119 constructs elicited high levels of antibodies to MSP119 but only one construct, the 19-kDa fragment expressed as a secreted fusion protein fromSaccharomyces cerevisiae (yP30P2MSP119), induced a high degree of protective immunity in Aotus nancymai against lethal P. falciparum challenge. Protective formulation required Freund's adjuvant; vaccination with yP30P2MSP119 in six other adjuvants that are suitable for human use induced lower levels of antibody response and no protection. These results emphasize the need to continue the search for an adjuvant that is comparable to Freund's adjuvant in potency and is safe for use in humans.

Antigen-specific antibodies (Abs) to the 19-kDa carboxy-terminal region of Plasmodium falciparum merozoite surface protein 1 (MSP1 19 ) play an important role in protective immunity to malaria. Mouse monoclonal Abs (MAbs) 12.10 and 12.8 recognizing MSP1 19 can inhibit red cell invasion by interfering with MSP1 processing on the merozoite surface. We show here that this ability is dependent on the intact Ab since Fab and F(ab) 2 fragments derived from MAb 12.10, although capable of binding MSP1 with high affinity and competing with the intact antibody for binding to MSP1, were unable to inhibit erythrocyte invasion or MSP1 processing. The DNA sequences of the variable (V) regions of both MAbs 12.8 and 12.10 were obtained, and partial amino acid sequences of the same regions were confirmed by mass spectrometry. Human chimeric Abs constructed by using these sequences, which combine the original mouse V regions with human ␥1 and ␥3 constant regions, retain the ability to bind to both parasites and recombinant MSP1 19 , and both chimeric human immunoglobulin G1s (IgG1s) were at least as good at inhibiting erythrocyte invasion as the parental murine MAbs 12.8 and 12.10. Furthermore, the human chimeric Abs of the IgG1 class (but not the corresponding human IgG3), induced significant NADPHmediated oxidative bursts and degranulation from human neutrophils. These chimeric human Abs will enable investigators to examine the role of human Fc␥ receptors in immunity to malaria using a transgenic parasite and mouse model and may prove useful in humans for neutralizing parasites as an adjunct to antimalarial drug therapy.

T cell production by the thymus, thymic size, cellularity and output all decrease drastically after puberty. Among the candidates that may mediate this decrease are the sex steroids: Hypersecretion or pharmacological administration of these hormones has long been known to induce thymic hypocellularity, and their depletion yields thymic hypercellularity. Here we show that a typical sex steroid, testosterone, specifically targets CD8+CD4+ double‐positive (DP) thymocytes for apoptosis via TNF‐α Anti‐TNF‐α monoclonal antibodies abrogated testosterone‐induced DP apoptosis, and TNF‐a– / – DP thymocytes were largely resistant to testosterone‐mediated apoptosis in vivo. Testosterone accomplishes this effect by up‐regulating TNF‐α production and by simultaneously sensitizing DP thymocytes to TNF‐α. Thus, TNF‐α is the critical mediator of sex steroid‐induced apoptosis in thymocytes, and its manipulation should provide a point of intervention to modulate T cell production in sex hormone disorders.

Apoptosis is one of the key regulatory mechanisms in tissue modeling and development. In the thymus, 95–98% of all thymocytes die by apoptosis because they failed to express a TCR with an optimal affinity for the selecting intrathymic peptide-MHC complexes. We studied the possible role of two prominent nerve growth factor (NGF-TNF) family member systems, Fas ligand (FasL)-Fas receptor (FasR) and TNF-α-TNFR, in apoptosis of murine CD8+4+ double-positive (DP) thymocytes induced via TCR-CD3- and cAMP-mediated signaling. TCR-CD3ε-mediated apoptosis of DP thymocytes was found not to be dependent on either of the two systems. The FasL-FasR system was also found to be dispensable for the cAMP-mediated apoptosis. By contrast, cAMP agonists (dibutyryl-cAMP and forskolin) induced apoptosis via TNF-α, as evidenced by 1) the ability of anti-TNF-α mAbs to abrogate cAMP analogue-induced DP apoptosis in a dose-dependent manner; and 2) increased resistance of DP thymocytes from TNF-α−/− and TNFR I−/−II−/− animals to cAMP agonist-mediated apoptosis. cAMP agonists induced DP thymocyte death by a combination of two mechanisms: first, they induced selective up-regulation of TNF-α production, and, second, they sensitized DP thymocytes to TNF-α. The latter effect may be due to the down-regulation of TNFR-associated factor 2 protein. These results identify TNF-α as the critical mediator of cAMP-induced apoptosis in thymocytes and provide a molecular explanation for how the cAMP stimulators, including the sex steroids, may modulate T cell production output, as observed under physiological and pharmacological conditions.

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