Granzyme B cleavage of autoantigens in autoimmunity

Abstract

The systemic autoimmune diseases are a complex group of disorders characterized by elaboration of high titer autoantibodies and immune-mediated damage of tissues. Two striking features of autoimmune rheumatic diseases are their self-sustaining nature and capacity for autoamplification, exemplified by disease flares. These features suggest the presence of a feed-forward cycle in disease propagation, in which immune effector pathways drive the generation/release of autoantigens, which in turn fuel the immune response. There is a growing awareness that structural modification during cytotoxic granule-induced cell death is a frequent and striking feature of autoantigens, and may be an important principle driving disease. This review focuses on granzyme B (GrB)-mediated cleavage of autoantigens including (i) features of GrB cleavage sites within autoantigens, (ii) co-location of cleavage sites with autoimmune epitopes, and (iii) GrB sensitivity of autoantigens in disease-relevant target tissue. The mechanisms whereby GrB-induced changes in autoantigen structure may contribute to the initiation and propagation of autoimmunity are reviewed and reveal that GrB has the potential to create or destroy autoimmune epitopes. As there remains no direct evidence showing a causal function for GrB cleavage of antigens in the generation of autoimmunity, this review highlights important outstanding questions about the function of GrB in autoantigen selection.

Figure 1. Cleavage sites in autoantigens are enriched in amino acids specifically preferred by GrB over group III caspases

(A) Specificities of GrB and group III caspases are similar but distinct. A comparison of the P2, P3, and P4 position requirements for a group III caspase (caspase-9) and GrB reveals differences in preferred amino acids (modified from Thornberry et al (22)). (B) Cleavage sites of autoantigens known to be GrB substrates are enriched in amino acids that are preferred by GrB in the P2 and P3 positions over caspase-9. Amino acids preferred by GrB are shown in green for the P2 and in blue for the P3 positions. Amino acids are indicated as follows: A,alanine; R,arginine; N, asparagine; D, aspartic acid; Q, glutamine; E, glutamic acid; G, glycine; H, histidine; I, isoleucine; L, leucine; M, methionine; K, lysine; P, proline; S, serine; T, threonine; V, valine.

Figure 2. Cleavage of GrB-sensitive forms of autoantigens may reveal cryptic epitopes in disease relevant target tissue

GrB-sensitive forms of autoantigens may be present disease-relevant target tissue due to unique intermolecular interactions or post-translational modifications (*) exposing GrB cleavage sites (red star). Seemingly ubiquitously expressed autoantigens may be susceptible to GrB cleavage (black triangle) only when in tissue-specific immunogenic conformations and may drive the generation of autoimmunity to cryptic epitopes (yellow).

Figure 3. Cleavage of autoantigens at GrB cleavage sites located in unstructured loops may liberate cryptic epitopes derived from structural elements

(A) Proteolysis by GrB (black triangle) occurring in unstructured loops or linker regions of autoantigens (red star), may enhance presentation of cryptic epitopes derived from adjacent structural elements (blue). (B) GrB cleavage may also induce structural changes leading to increased presentation of cryptic epitopes derived from previously hidden regions of autoantigens (green).

Figure 4. Autoimmune epitopes are co-located with GrB cleavage sites

B (blue) and T (yellow) cell epitopes of several autoantigens are clustered in regions containing GrB cleavage sites (red). Overlapping B and T cell epitopes (green) in La and U1-70kDa suggest autoimmune “hot spots” in these regions of antigens containing GrB cleavage sites.