Fine Specificity of Neonatal Lymphocytes to an Abundant Malaria Blood-Stage Antigen: Epitope Mapping ofPlasmodium falciparumMSP133

Abstract

Cord blood T cells have been reported to respond to a variety of exogenous Ags, including environmental allergens and various viruses and parasites, as demonstrated by enhanced proliferation and cytokine secretion. This finding is evidence that Ags in the maternal environment transplacentally prime and result in fetal development of memory T cells. Some studies suggest these neonatal T cell responses may arise by nonspecific activation of T cells that express TCRs with low binding affinity, thus lacking fine lymphocyte specificity. To address this question, we examined malaria Ag stimulation of human cord and adult blood mononuclear cells in samples from residents of a malaria endemic area in Kenya. We constructed overlapping 18-mer peptides derived from sequences contained in dimorphic alleles of the C-terminal 33-kDa fragment of Plasmodium falciparum merozoite protein 1. This study identified a dominant T cell epitope for one MSP133 allele (MAD20) and two T cell epitopes for the second allele (K1); these epitopes were nonoverlapping and allele specific. In a given donor, peptide-specific proliferation and IFN-γ secretion were highly concordant. However, IL-10 and IL-13 secretion were not correlated. Importantly, the fine specificity of lymphocyte proliferation and cytokine secretion in cord and adult blood mononuclear cells was similar. Cord blood cells obtained from malaria-infected pregnant women were 4-fold more likely to acquire a peptide-specific immune response. We conclude that the fetal malaria response functions in a fully adaptive manner and that this response may serve to help protect the infant from severe malaria during infancy.

Human Cord Blood CD4+CD25hi Regulatory T Cells Suppress Prenatally Acquired T Cell Responses toPlasmodium falciparumAntigens

“…Substantial amounts of IL-10 are produced by CBMC in malariaexposed fetuses (14,15,17,38), which are thought to be important for immunoregulation (36,38,71). This can occur both spontaneously and in response to malaria blood-stage Ags, suggesting that expansion of IL-10-producing T cells may be important for modulating malaria Ag-specific immune responses.…”

“…The nature of the initial exposure to malaria Ags likely affects the potentially diverse roles assumed by T regs in malaria infection. For some individuals, this first experience appears to occur in utero (10,12,14,17,83,84). This may have an important impact on the subsequent development of an individual's immune response to malaria and potentially to other Ags.…”

“…Lymphocytes were stimulated in separate cultures under the following conditions: 1) with highly purified MSP-1 42 (5 mg/ml, kindly provided by Carole Long and David Narum at Malaria Vaccine Development Unit, National Institutes of Health); 2) with PfP0 N1, N2, and C1 peptides (10 mg/ml); 3) with either anti-CD3/28 beads (one bead/five CD4 + T cells) or PHA (10 mg/ml) as a positive control; and 4) with medium alone (negative control). Optimal concentrations had been determined in previous studies and pilot experiments (14,15). Neutralizing anti-IL-10 and/or anti-TGF-b were added to a subset of samples to assess the role of immunosuppressive cytokines (n = 12; samples were selected based on available number of cells).…”

“…Lymphocyte proliferation was performed as previously described (14). Samples were performed in triplicate.…”

“…The accumulation of infected erythrocytes in the placenta may result in transplacental transport of infected erythrocytes or their soluble components, thereby exposing and sensitizing the fetal immune system to P. falciparum Ags (8)(9)(10)(11)(12). The reported frequency of malaria bloodstage-specific T and B cell responses in cord blood mononuclear cells (CBMC) ranges from ∼5% to .70% (13)(14)(15)(16)(17)(18). The consequences of this prenatal exposure of the infant to P. falciparum remain poorly understood.…”