Nature Reviews Immunology, Volume 8, December 2008

Self Tolerance “Ability to distinguish self-reactive from nonself- reactive lymphocytes”

Central Tolerance/Negative Selection Negative selection of self-reactive thymocytes mTECs expressing TSA AIRE turns on (peripheral) genes in the thymus Deletion of self-reactiveT cells

APECED Autoimmune polyendocrinopathy candidiasis ectodermal dystrophy; (APS-1) Monogenic autoimmune syndrome; mutations in the AIRE gene More common in certain population; Finnish & Sardinians Presence of autoantibodies specific for multiple self antigens  structural & functional similarities Lymphocytic infiltration of endocrine glands Autoantibodies can be used as diagnostic markers Most common mutation of AIRE: R257X in exon 6 / 13 base-pair deletion in exon 8 AIRE-deficient mice not susceptible to candidiasis

Common autoimmune diseases

What is the molecular mechanism of AIRE to prevent autoimmunity?

AIRE deficiency results in autoimmunity Infiltration of Peripheral tissues in AIRE -/- mice by mononuclear cells mTECs in AIRE -/- mice express less peripheral TSAs  Ins2 & Spt1 Other factors are also necessary for the efficient TSA expression AIRE -/- mice expressing transgenic HEL & TCRs developed diabetes AIRE -/- develop AI uveitis (IRBP) and gastritis (MUC6) AIRE -/- mice reacted also against AIRE-independent expressed proteins AIRE might have other roles in negative selection  chemokine or antigen presentation downregulated AIRE deficiency leads to an accumulation of imma. CD4 + Liston et al., 2003

AIRE expression by mTECs mTEC subpopulations express AIRE in the thymus  CD80, CD86, CD40 and MHC class II hi Two subsets of mTECs in the thymic medulla –CD80 low MHC classII low & CD80 hi MHC classII hi AIRE expression activates the promiscuous expression of TSAs in mature mTECs (CD80 hi MHC classII hi ) AIRE directs mTECs differentiation High level of heterogeneity between mTEC subpopulations mTEC differentiation: CD80 low AIRE -  CD80 hi AIRE -  CD80 hi AIRE + Defects in alternative NFkB pathways disturb AIRE expression RANK is needed for differentiation of mTECs from AIRE - to AIRE + cells

Functional domains of AIRE Domains characteristic of transcriptional regulators & chromatin-binding proteins Nuclear protein; fibrillar structures in the cytoplasm Functional domains are highly conserved between mammalian AIRE homologues Pitkanen et al., 2001 COS-1

Localization of nuclear bodies AIRE locates in dot-like structures AIRE nuclear body localization depends on the CARD domain PML nuclear bodies contain AIRE interacting proteins PML and AIRE Nuclear bodies are associated with the nuclear matrix AIRE nuclear bodies might influence nuclear organization of chromatin AIRE also in the nucleoplasm Bjorses et al., 1999 COS-1 HeLa

Transcriptional activation AIRE is a potent transcriptional activator PHD zinc fingers, CARD and SAND participate in transcriptional activation of AIRE AIRE does not seems to act as a specific DNA binding transcription factor, but rather have a more general function AIRE binds directly to CBP  promotes the transcription PIAS1 and DNA-PK identified as protein partners of AIRE  interaction are needed for the formation of AIRE associated nuclear structures and for the regulation of AIRE-mediated gene expression PIAS1 and AIRE might interact via different nuclear bodies DNA-PK phosphorylates N-terminal region of AIRE

Transcriptional elongation and histone binding Trimethylated H3K4 genes are active and trimethylated H3K27 are silent; both modification are poised for transcription TSA genes lacking trimethylated H3K4  silent genes AIRE preferentially binds to unmodified histones AIRE promotes transcriptional elongation  via P-TEFb P-TEFb forms a complex with RNA-Pol. II & removes N-TEF PHD zinc fingers binds to N- terminal tail of unmythyl. H3K4 AIRE functions as a transcription activator and initiator of gene expression

Coordinated gene regulation? AIRE might function as an activator & repressor AIRE functions as a co-regulator of genomic clusters Together with transcriptional regulators AIRE functions in protein complexes

Stochastic gene regulation? AIRE activates genes in a stochastic manner  identical cell clones show variations in protein expression between individual cells Influencing factors are varying amounts of gene regulators, availability of chromosomal positions etc. AIRE regulated genes are expressed in mTECs in random combination and at relatively low levels

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