Hyper-targeted T-cell therapies

What can we learn from treating other autoimmune diseases? Will we be able to prove that MS is an antigen-specific T-cell mediated autoimmune disease?

Proving a disease is autoimmune is hard. Theoretical immunologists use Witebsky’s postulates, which claim a disease is autoimmune if (i) the clinical disease or phenotype can be reproduced through the transfer of autoantibodies and/or lymphocytes to animals or inadvertently to humans, (ii) the disease can be reproduced in experimental animal models and (iii) autoreactive T cells or autoantibodies are identified. This definition has worked relatively well for antibody-mediated disorders with well-defined auto-antigens, such as myasthenia gravis with anti-acetylcholine receptor antibodies or anti-aquaporin-4 positive neuromyelitis optic. 

How many swallows does it take to make a summer?

However, in putative T-cell-mediated diseases, these postulates are hard to fulfil. This is because T-cells are promiscuous and can bind to and react to many different antigen fragments or peptides presented to them by so-called antigen-presenting cells.  In MS, many different peptide fragments from several putative autoantigens have been identified, but none of them has been proven to drive MS disease activity. 

Immunology is complicated, but I will explain things as simply as possible. When you make new T-cells in the thymus, you must first select which T-cells are safe to let out of the thymus or T-cell nursery (T is for thymus). Only T-cells that pass positive and negative selection can enter the peripheral circulation. 

To ensure reactivity and specificity, positively selected thymocytes must interact with several cell surface molecules, called the major histocompatibility complex (MHC). In humans, these are called the HLA molecules (human leukocyte antigens). Positive selection selects cells with a T cell receptor able to bind MHC class I or II molecules with at least a weak affinity. This weak affinity gives them a pro-survival signal through their T-cell receptor (TCR) and keeps them alive. Cells with TCRs that can’t bind MHC, die by a cell death programme called apoptosis. The rationale for positive selection is to have a population of cells that can least read the messages presented to them via antigen-presenting cells. These young T-cells then undergo negative selection, which deletes T cells with high affinity for self-peptides via apoptosis, thus ensuring self-tolerance. 

The thymus educates T-cells by presenting to them as many peptides as possible from the human proteome (all the proteins encoded by the human genome). However, not all peptides can be presented to all T-cells in this process; hence, there is a chance that potentially self-reactive T-cells are not purged from the repertoire and are released from the thymus. 

Please be aware that there are also peripheral tolerance mechanisms, which are context-specific, that keep potentially self-reactive T-cells anergic or tolerant of self. When T-cells see the antigens that bind strongly to their TCRs in a non-inflammatory setting with inhibitory signals, they remain tolerant and don’t react (anergic). Conversely, if they see their antigen in the context of inflammation and receive the necessary co-stimulatory or danger signals, they are activated and orchestrate an inflammatory reaction. It is presumed that the activation of potentially autoreactive T-cells is more likely to occur in response to an infection, i.e. components of the infection act as the danger signals. If a protein or antigen from the infectious agents looks very similar to a human or self-protein and is interpreted by the immune system as foreign, this can activate autoreactive T-cells. The latter is called molecular mimicry and is the dominant mechanism of how infectious agents trigger autoimmunity. 

Molecular mimicry is the dominant hypothesis to explain how EBV drives MS disease activity. Despite a long list of potential autoantigens (see below), the MS research community have yet to find a dominant clone or clones of T-cells that are driving MS. If you could identify the specific autoreactive clones, then you could target them with a specific therapy and treat or even cure someone with MS. This is clearly easier said than done. 

This is why a case report of someone with ankylosing spondylitis treated using this strategy has made the autoimmune community sit up and notice. Ankylosing spondylitis is considered an autoimmune disease where the immune system attacks the sacroiliac and joints of the spine. It is a very debilitating disease. From an immunological perspective, it has one of the strongest associations between an MHC or HLA variant of all the known putative autoimmune diseases, i.e. HLA-B27. The association is so strong that HLA-B27 is almost part of the diagnostic criteria, i.e. ~95% of people with ankylosing spondylitis (pwAS) are HLA-B27-positive. In comparison, the strongest HLA association in MS is with  DRB1*1501, with only 60-70% of pwMS carrying this variant. 

In the study below, the investigators found a dominant CD8+ve T-cell clone that expressed the T-cell receptor beta variable 9 (TRBV9), which interacted with the HLA-B27 in a person with ankylosing spondylitis. In a separate study, TRBV9 has been shown to react to pathogenic peptides from bacteria (Chlamydia, Klebsiella, Salmonella, Shigella and Yersinia species) and peptides from many putative autoantigens in patients with ankylosing spondylitis (see paper 2 below). The patient was treated with a monoclonal antibody that selectively depleted the family of T-cells expressing TRBV9 and, as a result, went into remission. His disease relapsed when the antibody treatment wore off, and the TRBV9+ T-cells reappeared. His condition then responded to an additional course of treatment, which has now been continued. 

Figure from Britanova et al. Targeted depletion of TRBV9+ T cells as immunotherapy in a patient with ankylosing spondylitis. Nat Med. 2023 Nov;29(11):2731-2736.

This n=1 trial would indicate that the treatment was working and the TRBV9+ T-cells are pathogenic in this patient. I know this is only one patient, but one patient is enough of a proof-of-concept (n=1 trial) to take this strategy forward as a potential treatment for ankylosing spondylitis. Let’s hope this case report ushers in a new era of precision-targeted immunotherapies for people with T-cell-mediated autoimmune diseases. 

So, how easy will it be to translate this type of treatment into MS? 

At present, this isn't easy. I am yet to be convinced that any one of the putative autoantigens associated with MS is pathogenic and is directly involved in the pathogenesis of the disease. However, this is a story for another time. In short, several trials have been done using altered peptide ligands to try tolerise putative autoreactive T-cells. These studies didn’t work, possibly because they targeted the incorrect autoantigen. 

What about using the MS-associated HLA variants as bait to identify autoantigens? This has been done directly and indirectly in the past and helped generate a long list of potential MS autoantigens: 

1. Myelin basic protein (MBP) (Wucherpfennig, 2001)

2. Myelin oligodendrocyte glycoprotein (MOG) (Quagliata et al., 2023)

3. Proteolipid protein (PLP) (Olson et al., 2004; Owens et al., 2023)

4. Myelin-associated glycoprotein (MAG) (Jakhmola et al., 2022)

5. Myelin-associated oligodendrocytic basic protein (MOBP) (Kaushansky et al., 2010)

6. 2',3'-Cyclic-nucleotide 3'-phosphodiesterase (CNPase) (Walsh and Murray, 1998)

7. Anoctamin 2 (ANO2) (Ayoglu et al., 2016)

8. GDP-L-fucose synthase (GDP-L-FS) (Planas et al., 2018)

9. RAS guanyl-releasing protein 2 (RASGRP2) (Jelcic et al., 2018)

10. Glial cell adhesion molecule (GlialCAM) (Lanz et al., 2022)

11. Fatty Acid Binding Protein 7 (FABP7) (Bronge et al., 2022)

12. Prokineticin 2 (PROK2)(Bronge et al., 2022)

13. Reticulon 3 (RTN3)(Bronge et al., 2022)

14. Synaptosome Associated Protein 91 (SNAP91)(Bronge et al., 2022)

15. αB-crystallin (CRYAB) (Bronge et al., 2022; van Noort et al., 2023)

16. Septin 9 (SEPT9) (Lindsey, 2017)

17. Transaldolase (TALDO1) (Colombo et al., 1997)

18. S100ß protein (PS100ß) (Dornmair et al., 2009)

19. ßsynuclein (SNCB) (Lodygin et al., 2019)

But as I have alluded to above, more work is needed to show these autoantigens are pathogenic. 

Please be aware that DRB1*1501, the major MS risk gene, is usually inherited on a stretch of DNA with other HLA variants. The extended haplotype is DRB5*0101- DRB1*1501-DQA1*0102- DQB1*0602. This co-inheritance is called linkage dysequilibrium, which means these genetic variants tend to be inherited together. Therefore, it may not necessarily be DRB1*1501 that is driving MS but another HLA variant linked to DRB1*1501. The story gets even more complicated in MS in that some HLA variants protect you from getting MS, and this protection dominates over the at-risk variants. How this dominance of protective HLA alleles works from a biological perspective in MS is unknown. This is why using at-risk HLA alleles as bait to find pathogenic T-cell clones in MS is not that simple and not as simple as in this case of ankylosing spondylitis. This does not mean the MS community should give up on antigen-specific therapies or try to find hyper-selective T-cell targeted treatments. Testing these strategies is part of the scientific process of proving or disproving MS as a T-cell-mediated autoimmune disease. 

Please note we are still using immunological sledgehammers to treat MS, i.e. taking out or suppressing large parts of the immune system. Finding a precision-targeted therapy, for example, by depleting a single family of T-cells expressing a particular TCR, will make our treatments safer in the future.

I know this Newsletter is very scientific, but I want to point out that high-brow immunological research and treatments are based on detailed and precise scientific insights. I hope this newsletter will generate many questions. 

Paper 1

Britanova et al. Targeted depletion of TRBV9+ T cells as immunotherapy in a patient with ankylosing spondylitis. Nat Med. 2023 Nov;29(11):2731-2736.

Autoimmunity is intrinsically driven by memory T and B cell clones inappropriately targeted at self-antigens. Selective depletion or suppression of self-reactive T cells remains a holy grail of autoimmune therapy, but disease-associated T cell receptors (TCRs) and cognate antigenic epitopes remained elusive. A TRBV9-containing CD8+ TCR motif was recently associated with the pathogenesis of ankylosing spondylitis, psoriatic arthritis and acute anterior uveitis, and cognate HLA-B*27-presented epitopes were identified. Following successful testing in nonhuman primate models, here we report human TRBV9+ T cell elimination in ankylosing spondylitis. The patient achieved remission within 3 months and ceased anti-TNF therapy after 5 years of continuous use. Complete remission has now persisted for 4 years, with three doses of anti-TRBV9 administered per year. We also observed a profound improvement in spinal mobility metrics and the Bath Ankylosing Spondylitis Metrology Index (BASMI). This represents a possibly curative therapy of an autoimmune disease via selective depletion of a TRBV-defined group of T cells. The anti-TRBV9 therapy could potentially be applicable to other HLA-B*27-associated spondyloarthropathies. Such targeted elimination of the underlying cause of the disease without systemic immunosuppression could offer a new generation of safe and efficient therapies for autoimmunity.

Paper 2

Yang et al. Autoimmunity-associated T cell receptors recognize HLA-B*27-bound peptides. Nature. 2022 Dec;612(7941):771-777. 

Human leucocyte antigen B*27 (HLA-B*27) is strongly associated with inflammatory diseases of the spine and pelvis (for example, ankylosing spondylitis (AS)) and the eye (that is, acute anterior uveitis (AAU))1. How HLA-B*27 facilitates disease remains unknown, but one possible mechanism could involve presentation of pathogenic peptides to CD8+ T cells. Here we isolated orphan T cell receptors (TCRs) expressing a disease-associated public β-chain variable region-complementary-determining region 3β (BV9-CDR3β) motif2-4 from blood and synovial fluid T cells from individuals with AS and from the eye in individuals with AAU. These TCRs showed consistent α-chain variable region (AV21) chain pairing and were clonally expanded in the joint and eye. We used HLA-B*27:05 yeast display peptide libraries to identify shared self-peptides and microbial peptides that activated the AS- and AAU-derived TCRs. Structural analysis revealed that TCR cross-reactivity for peptide-MHC was rooted in a shared binding motif present in both self-antigens and microbial antigens that engages the BV9-CDR3β TCRs. These findings support the hypothesis that microbial antigens and self-antigens could play a pathogenic role in HLA-B*27-associated disease.

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Please note that the opinions expressed here are those of Professor Giovannoni and do not necessarily reflect the positions of Queen Mary University of London or Barts Health NHS Trust. The advice is intended as general and should not be interpreted as personal clinical advice. If you have problems, please tell your healthcare professional, who will be able to help you.