Significance of transporter associated with antigen processing gene polymorphism in living related renal transplantation

Monitoring indirect presentation of alloantigens by utilizing the autologous processing machinery of dendritic cells in-vitro

BACKGROUND

Judicious individualization of immunosuppression protocols requires the ability to monitor the recipients' specific immune response towards the allograft. While several indirect presentation studies have been reported, currently there are no consistently reliable means to assess the state of transplant acceptance. A potential explanation might be the lack of established kinetics for in-vitro indirect presentation assays.

METHODS

Dendritic cell (DCs) were used as self-specific "processing machinery" for the generation of allogeneic peptide repertoire tailored by the individual's capacity to process antigens via the indirect pathway. The kinetics of antigen processing and presentation of newly acquired antigens was then assessed to define the following: (1) Optimal time to introduce apoptotic-allogeneic cells to DCs; (2) optimal time for processing before induction of DC maturation; and (3) optimal time for stimulation of autologous T cells by the pulsed DCs.

RESULTS

Altering kinetic parameters associated with time allotted for antigen processing; antigen presentation and DCs maturation, showed substantial variations in the level of T cell activation, as was documented by mixed lymphocyte reaction (MLR)-like proliferation assays. Further experiments using allogeneic cellular extracts showed similar variations in T cell stimulation by monitoring T cell precursor frequencies and interferon (IFN)-gamma secretion ELISPOT assays. Using the appropriate kinetic parameters, we have also shown difference in T-cell-subsets (CD4, CD8) precursor frequencies and cytokine secretion in response to different antigenic sources.

CONCLUSIONS

The ability to construct an in-vitro, recipient-tailored, donor-specific, indirect presentation assay is dependent on essential kinetic parameters associated with antigen processing and presentation, as described in this report. Moreover, the use of this approach may circumvent the need for identifying specific immunogenic peptides for cancer-related or other vaccine-development studies.

The transporter associated with antigen processing: function and implications in human diseases

The adaptive immune systems have evolved to protect the organism against pathogens encountering the host. Extracellular occurring viruses or bacteria are mainly bound by antibodies from the humoral branch of the immune response, whereas infected or malignant cells are identified and eliminated by the cellular immune system. To enable the recognition, proteins are cleaved into peptides in the cytosol and are presented on the cell surface by class I molecules of the major histocompatibility complex (MHC). The transport of the antigenic peptides into the lumen of the endoplasmic reticulum (ER) and loading onto the MHC class I molecules is an essential process for the presentation to cytotoxic T lymphocytes. The delivery of these peptides is performed by the transporter associated with antigen processing (TAP). TAP is a heterodimer of TAP1 and TAP2, each subunit containing transmembrane domains and an ATP-binding motif. Sequence homology analysis revealed that TAP belongs to the superfamily of ATP-binding cassette transporters. Loss of TAP function leads to a loss of cell surface expression of MHC class I molecules. This may be a strategy for tumors and virus-infected cells to escape immune surveillance. Structure and function of the TAP complex as well as the implications of loss or downregulation of TAP is the topic of this review.

TAPI polymorphisms in several human ethnic groups: characteristics, evolution, and genotyping strategies

Genetic variations in the locus encoding the transporter associated with antigen processing, subunit 1 (TAP1), were systematically studied using samples from Caucasians, Africans, Brazilians, and compared with data from chimpanzees. PCR-amplified genomic sequences corresponding to the 11 exons were analyzed by single-strand conformation polymorphism (SSCP) and sequencing. Six nonsynonymous and 2 synonymous single nucleotide polymorphisms (SNPs) were found to be common in one ethnic group or another, and they involved codons 254 (Gly-GGC/Gly-GGT) in exon 3, 333 (Ile-ATC/Val-GTC) in exon 4, 370 (Ala-GCT/Val-GTT) in exon 5, 458 (Val-GTG/Leu-TTG) in exon 6, 518 (Val-GTC/Ile-ATC) in exon 7, 637 (Asp-GAC/Gly-GGC), 648 (Arg-CGA/Gln-CAA) and 661 (Pro-CCG/Pro-CCA) in exon 10. At each SNP site the sequence listed first was predominant in all ethnic groups. Several SNPs segregated on the same chromosome regardless of populations and species. Together, the SNPs produced 5 major human TAP1 alleles, 4 of which matched the officially recognized alleles *0101, *02011, *0301, and *0401; the 5th allele differed from each of those by at least 4 SNPs. Overall, TAP1*0101 was the predominant allele in all ethnic groups, with frequencies ranging from 0.667 in Zambians to 0.808 in US Caucasians. The TAP1*0401 frequency showed the greatest difference between Africans (0.221-0.254) and Caucasians (0.033), with Brazilians (0.058) fitting in the middle. Consistent with earlier work based on Caucasians and gorillas, *0101 appeared to be the newest human TAP1 allele, suggesting a dramatic spread of *0101 into all human populations examined. Characterization of TAP1 polymorphisms allowed the design of a PCR-based genotyping scheme that targeted 7 SNP sites and required 2 separate genotyping techniques.