No abstract

To investigate the role of CD4+CD25+Foxp3+ regulatory T (Treg) cells in blood-stage malaria, we compared Plasmodium chabaudi AS infection in wild-type (WT) C57BL/6 and transgenic mice overexpressing the transcription factor Foxp3 (Foxp3Tg) and observed that Foxp3Tg mice experienced lethal infection and deficient malaria-specific immune responses. Adoptive transfer of total CD4+ T cells from Foxp3Tg mice or CD4+CD25+ T cells from WT mice to naive WT recipients confirmed that high numbers of Treg cells compromised immune control of malaria. Transfer of GFP+CD4+CD25+ T cells to naive WT recipients together with immunohistochemical staining of spleens from infected WT mice demonstrated that Foxp3+ Treg cells localized in the T cell area of the spleen. Determination of CD4+Foxp3+ Treg cell responses in the spleen of infected WT mice revealed a significant but transient increase in CD4+Foxp3+ Treg cells early in infection. This was followed by a significant and sustained decrease due to reduced proliferation and apoptosis of CD4+Foxp3+ Treg cells. Importantly, the kinetics of IL-2 secretion by effector CD4+Foxp3− T cells coincided with changes in CD4+Foxp3+ cells and the differentiation of CD4+T-bet+IFN-γ+ cells required for immune control of infection. Administration of the IL-2/anti–IL-2 mAb (clone JES6-1) complex to infected WT mice increased the severity of P. chabaudi AS infection and promoted expansion of Foxp3+ Treg cells. Collectively, these data demonstrate that the ability to control and eliminate P. chabaudi AS infection is due to a tight balance between natural Treg cells and effector CD4+ Th1 cells, a balance regulated in part by IL-2.

The polymorphism at position beta69 of the human leukocyte antigen (HLA)-DP molecule has been associated with susceptibility to several immune disorders and alloreactivity. Using molecular modeling, we have predicted a detailed structure of the HLA-DP2 molecule (carrying Glubeta69) complexed with class II associated invariant chain derived peptide (CLIP) and compared it with the form carrying Lys at beta69 (HLA-DP2K69). Major changes between the two models were observed in the shape and charge distribution of pocket 4 and of the nearby pocket 6. Consequently, we analyzed in detail the peptide-binding specificities of both HLA-DP molecules expressed as recombinant proteins. We first determined that the minimum peptide-binding core of CLIP for both HLA-DP2 and DP2K69 is represented by nine aminoacids corresponding to the sequence 91-99 of invariant chain (Ii). We then assessed the peptide-binding specificities of the two pockets and determined the role of position beta69, using competition tests with the Ii-derived peptide CLIP and its mutated forms carrying all the aminoacidic substitutions in P4 and P6. Pocket 4 of HLA-DP2 showed high affinity for positively charged, aromatic, and polar residues, whereas aliphatic residues were disfavored. Pocket 4 of the DP2K69 variant showed a reduced aminoacid selectivity with aromatic residues most preferred. Pocket 6 of HLA-DP2 showed high affinity for aromatic residues, which was increased in DP2K69 and extended to arginine. Finally, we used the experimental data to determine the best molecular-modeling approach for assessing aminoacid selectivity of the two pockets. The results with best predictive value were obtained when single aminoacids were evaluated inside each single pocket, thus, reducing the influence of the overall peptide/ major histocompatibility complex interaction. In conclusion, the HLA-DPbeta69 polymorphism plays a fundamental role in the peptide-binding selectivity of HLA-DP. Furthermore, as this polymorphism is the main change in the pocket 4 area of HLA-DP, it could represent a supertype among HLA-DP molecules significantly contributing to the selection of epitopes presented in the context of this HLA isotype.