Since in Rheumatoid Arthritis the human body mistakenly attacks citrullinated proteins in the joints, vaccines that induce immune tolerance could prevent this attack. Instead of boosting the immune system, these vaccines aim to restore immune tolerance by teaching the immune system to recognize citrullinated proteins as "self" again, reducing inflammation without suppressing the entire immune system.
A study by Benham et al. (2015) explored the safety, biological, and clinical effects of autologous dendritic cells (DCs) modified with a nuclear factor κB (NF-κB) inhibitor exposed to four citrullinated peptide antigens, designated "Rheumavax," in a single-center, open-labeled, first-in-human phase 1 trial.
Rheumavax did not induce disease flares in patients recruited with minimal disease activity, and the Disease Activity Score in 28 joints (DAS28) decreased within 1 month in Rheumavax-treated patients with active disease. This exploratory study demonstrated the safety and biological activity of a single intradermal injection of autologous, citrullinated peptide-exposed DCs and provides a rationale for further studies to assess the clinical efficacy and antigen-specific effects of autoantigen immunomodulatory therapy in RA.
Rheumavax is a personalized vaccine that uses a patient's own dendritic cells modified with citrullinated peptides. Early trials showed that it could reduce inflammatory markers and improve symptoms. The goal of this type of vaccine is to achieve long-term remission by "re-educating" the T-cells that cause joint damage.
Kaminaka et al. (2019) investigated the adjuvant activity of various substances and found that citrulline can function as an adjuvant. Aluminum salts that are commonly used as adjuvants are not water soluble; therefore, some difficulties are encountered while using them as vaccine adjuvants. However, citrulline is easy to use because of its water solubility. The study revealed the adjuvant activity of citrulline by using viral antigens and amyloid β peptide. Water-soluble citrulline, which is present in our body, is a potential adjuvant candidate.
Brentville et al. (2020) reported that a combination of citrullinated vimentin and enolase peptides called Modi-1 stimulated strong CD4 T cell responses in mice. In the near future, medicinal researchers may be able to harness the Modi-1-induced immune response to citrullinated vimentin and redirect it to destroy cancer cells. Tumor recognition by the immune system depends on both citrullination and autophagy. To conclude, the Modi-1 citrullinated peptide vaccine induces potent CD4-mediated anti-tumor responses in mouse models. Since a CD4 T cell repertoire is present in ovarian cancer patients to the citrullinated peptides, Modi-1 could be an effective vaccine for ovarian cancer patients.
Table 1: Peptides Selected for Vaccination by Brentville et al. 2020.
| Peptides | Sequences | Notes |
| Vimentin aa28 to 49 | R-SYVTTST--R--TYSLGSAL--R--PSTS | For vaccination, peptides were linked via the N-terminus to TLR1/2 ligand AMPLIVANT. |
| Citrullinated | (Cit)SYVTTST(Cit)TYSLGSAL(Cit)PSTS |
| Vimentin aa415 to 433 | LPNFSSLNL--R--ETNLDSLP |
| Citrullinated | LPNFSSLNL(Cit)ETNLDSLPL |
| Enolase aa241 to 260 Citrullinated | VIGMDVAASEFF(Cit)SGKYDLD |
| Enolase aa241 to 260 | VIGMDVAASEFF--R--SGKYDLD |
Brentville et al. showed that citrullinated vimentin and enolase peptides can be combined into a single vaccine to stimulate a CD4 T cell response for an efficient tumor therapy.
Further, Han et al. (2023) showed that citrullinated fibrinogen complexes with serum amyloid A3 (SAA) are present in the pre-metastatic lung and that they facilitate tumor metastasis by interacting with circulating tumor cells. Autopsy samples analyzed showed depositions of citrullinated fibrinogen in the lungs. The antibody used in the study recognized citrullinated fibrinogen but not unmodified fibrinogen. A humanized mouse metastasis assay revealed that a synthetic citrullinated peptide prevented metastasis. Han et al. suggested that citrullinated fibrinogen is a potential target for diagnosing and preventing metastasis in early stages.
Pulmonary endothelial cells mediate the citrullination of fibrinogen, altering its conformation, surface charge, and binding properties with serum amyloid A proteins (SAAs), thereby forming a host tissue-derived metastatic pathogen. The human-specific SAAs-citrullinated fibrinogen complex recruits cancer cells to form a protein-metastatic cell aggregation in humanized SAA cluster mice. However, a citrullinated fibrinogen peptide acts as a competitive inhibitor to block the homing of metastatic cells into the SAAs-citrullinated fibrinogen sites.
The specific antibody used to visualize SAAs-citrullinated fibrinogen sites identified potential metastatic sites in the lungs of patients, suggesting that citrullinated peptides can act as a metastasis inhibitor.
Reference
Benham H, Nel HJ, Law SC, Mehdi AM, Street S, Ramnoruth N, Pahau H, Lee BT, Ng J, Brunck ME, Hyde C, Trouw LA, Dudek NL, Purcell AW, O'Sullivan BJ, Connolly JE, Paul SK, Lê Cao KA, Thomas R. Citrullinated peptide dendritic cell immunotherapy in HLA risk genotype-positive rheumatoid arthritis patients. Sci Transl Med. 2015 Jun 3;7 (290):290ra87. [PubMed]
Brentville VA, Metheringham RL, Daniels I, Atabani S, Symonds P, Cook KW, Vankemmelbeke M, Choudhury R, Vaghela P, Gijon M, Meiners G, Krebber WJ, Melief CJM, Durrant LG. Combination vaccine based on citrullinated vimentin and enolase peptides induces potent CD4-mediated anti-tumor responses. J Immunother Cancer. 2020 Jun;8(1):e000560. [PMC]
Han Y, Tomita T, Kato M, Ashihara N, Higuchi Y, Matoba H, Wang W, Hayashi H, Itoh Y, Takahashi S, Kurita H, Nakayama J, Okumura N, Hiratsuka S. Citrullinated fibrinogen-SAAs complex causes vascular metastagenesis. Nat Commun. 2023 Aug 24;14(1):4960. [PMC]
Kaminaka K, Matsuda J, Nozaki C. Citrulline as a novel adjuvant candidate for vaccines. Biomed Res. 2019;40(1):1-7. [Jstage]
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