My attempt on "Decoding the HLA" with tweetorial & some slides/VisualArts @kidney_boy @NSMCInternship @Nephro_Sparks @hswapnil @rheault_m @RenalFellowNtwk @ssfarouk #Transplant
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MHC complex
cluster of genes--> encodes HLA molecules
located on chromosome 6p21
over 200 genes; of which 40% involved in immune response
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HLA Genes:
main function is immune regulation
major barrier to transplant
gene products are histocompatibility molecules
highly polymorphic
significant linkage disequilibrium
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MHC-HLA genome-> divided into 3 regions
Class I: molecules present on nucleated cells
Class II: present on antigen presenting cells
Class III: are NOT histocompatibility genes
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HLA Class I
Classical: HLA-A, -B, -C antigens
Non-classical: HLA -E, -F, -G antigens
Class I like antigens: MICA, MICB
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HLA Class II
encodes HLA -DP, -DQ, -DR molecules
expressed on antigen presenting cells
presents antigens to CD8 T cells
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HLA Class III
not involved in histocompatibility
encodes complement proteins like C2, C4, cytokines
has both immune and non-immune genes
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HLA - Structure
Class I has heavy glycoprotein alpha chain bound to beta 2 microglobulin (gene on chromosome 15). Alpha chain has three domains and is transmembrane
Class II has both alpha & beta chains
peptide groove is highly polymorphic region
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HLA Class I Antigen Processing & Presentation
Altered self or/viral protein --> degraded intracellularly into peptides by proteases --> which fit into peptide groove--> transferred to cell surface to present to CD8 T cells
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HLA Class II Antigen Processing & Presentation:
Exogenous antigen---> endocytosis--> degraded into lysosomes--> combines with MHC compartment-->attached to peptide groove--> transferred to cell surface--> present to CD4 T cells
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HLA typing:
to assess the HLA genotype
historically, it was done with serological microcytotoxicity using viable lymphocytes & panel of anti-sera
Now, done with DNA-based HLA typing which is more reliable, precise & offers stable nomenclature
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HLA nomenclature
serological typing defines " antigens"
DNA typing defines "alleles"
WHO nomenclature includes distinct fields seperated by colon ( : )for antigen, allele, specific protein, nucleotide substitution in coding & non-coding region
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HLA Haplotype and Inheritance:
HLA alleles from each parent is called haplotype
it is inherited in Mandelian fashion
co-domminantly expressed
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HLA antibody
preformed antibodies against HLA antigens are major transplant barriers and has significant impact on allograft survival
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Strategies to detect HLA Abs
PRA: % panel reactive antibodies to assess sensitization in general
Identification of specific antibodies in recipient
crossmatch when kidney is available
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Crossmatch -->three methods
standard complement dependent cytotoxicity CDC (1960s)
Flow T & B cells crossmatch (1980s)
Solid phase technologies using single or multiple alleles-
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Virtual Crossmatch
predicts presence of donor specific antibodies using solid phase without an actual crossmatch
compares recipient's HLA ab to donor HLA type to predict Xmatch results
its use in lieu of 'wet' crossmatch is on the rise
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Summary
methods for high resolution HLA typing & precise quantitation of HLA ab in transplant on the rise
impetus for virtual Xmatch
need for better understanding of immunogenicity of HLA molecules & the role of ab on graft survival.
Thank you