Key antimicrobial peptides like hBD-2 and hBD-3 are abundant in psoriatic skin but completely absent in the joint tissue of psoriatic arthritis, helping explain why some psoriasis patients develop joint disease and others do not.
5 of 8 AMPs completely absent in joint tissuehBD-2, hBD-3, psoriasin, RNase 7, and LL-37 were found in psoriatic skin but undetectable in the synovial tissue of psoriatic arthritis, rheumatoid arthritis, or osteoarthritis
What the researchers found
Psoriatic skin showed broad, strong expression of all eight antimicrobial peptides tested, with particularly robust keratinocyte expression. In contrast, psoriatic arthritis synovial tissue only expressed S100 A8, S100 A9, and HNP1-3, and only in the synovium sublining layer — not in the lining layer.
Critically, hBD-2, hBD-3, psoriasin (S100 A7), RNase 7, and cathelicidin LL-37 were completely undetectable in the joint tissue of PsA, RA, or OA patients. Since hBD-2 is genetically linked to psoriasis susceptibility, its skin-only expression provides a molecular explanation for why psoriasis and psoriatic arthritis behave as distinct — though related — diseases.
Why it matters
This research provides a molecular explanation for one of dermatology's and rheumatology's key puzzles: why only about 30% of psoriasis patients develop psoriatic arthritis. By showing that skin-specific antimicrobial peptides like hBD-2 — which is genetically linked to psoriasis — are not expressed in joints at all, the study demonstrates that the innate immune environment differs fundamentally between these two tissues. This could eventually inform diagnostic tools that predict joint disease risk and guide tissue-specific treatment strategies.
How the study worked
The study examined knee biopsies from 22 patients (10 with psoriatic arthritis, 8 with rheumatoid arthritis, and 4 with osteoarthritis), plus lesional and non-lesional skin biopsies from 4 psoriasis patients and 4 healthy controls. Immunohistochemistry was performed using antibodies against eight AMPs: S100 A8, S100 A9, HNP1-3, hBD-2, hBD-3, cathelicidin LL-37, psoriasin, and RNase 7. Expression was scored semi-quantitatively across tissue compartments.
What this study cannot tell us
The sample sizes are small (22 joint biopsies, 8 skin biopsies), which limits statistical power. Semi-quantitative immunohistochemistry scoring is subjective and less precise than quantitative methods like ELISA or mass spectrometry. The study only examined knee joints, so findings may not generalize to other affected joints. Skin biopsies were from different patients than joint biopsies, preventing direct within-patient comparison.
How to read the evidence
This is a cross-sectional observational study using tissue biopsies from a relatively small number of patients. While the immunohistochemistry approach is well-established, the semi-quantitative scoring and small sample sizes limit the strength of conclusions. The case-control design provides useful comparative data but cannot establish causation.
When this study was published
Published in 2016, this study is about a decade old. The fundamental findings about tissue-specific AMP expression remain relevant, though newer techniques like single-cell RNA sequencing have since provided more detailed views of immune cell heterogeneity in these tissues.
The bigger picture
Antimicrobial peptides are increasingly recognized not just as infection fighters but as key regulators of inflammation and immune signaling. This study sits at the intersection of dermatology, rheumatology, and innate immunity, showing that tissue-specific peptide expression patterns may determine which organs are affected in autoimmune disease. The findings align with a broader shift toward understanding psoriatic disease as a spectrum with tissue-specific immune mechanisms rather than a single uniform condition.
Questions still open
- Could measuring AMP expression levels in psoriasis skin predict which patients will later develop psoriatic arthritis?
- What drives the tissue-specific expression patterns — is it the cell types present, local cytokine environments, or epigenetic regulation?
- Would therapeutic modulation of specific AMPs like hBD-2 in skin affect psoriasis severity or arthritis risk?
Common questions
What are antimicrobial peptides, and what do they have to do with psoriasis?
Does this study explain why some psoriasis patients get arthritis and others don't?
Read the original research
Differential expression of antimicrobial peptides in psoriasis and psoriatic arthritis as a novel contributory mechanism for skin and joint disease heterogeneity.
Scandinavian journal of rheumatology, 45(3), 188-96
Citation
Bierkarre, H; Harder, J; Cuthbert, R; Emery, P; Leuschner, I; Mrowietz, U; Hedderich, J; McGonagle, D; Gläser, R. (2016). Differential expression of antimicrobial peptides in psoriasis and psoriatic arthritis as a novel contributory mechanism for skin and joint disease heterogeneity.. Scandinavian journal of rheumatology, 45(3), 188-96. https://doi.org/10.3109/03009742.2015.1091497