Summary

Correction of lower-limb alignment with HTO attenuated symptoms and synovial inflammation via changing biological intra-articular microenvironment of osteoarthritic (OA) knees accompanying synovial macrophage polarization from M1 to M2 phenotype.

Abstract

Background

In recent years, accumulating evidence suggests that synovial inflammation is associated with knee symptoms and progressive OA. Although biomechanical findings have shown that HTO decreases medial compartment loading, whether inflammation and biological microenvironment are altered by optimizing knee alignment remains unclear. Both biological and biomechanical improvements would be ideal to achieve the “remission” of progressive knee OA.

Objective

This study aimed to determine whether alignment correction by high tibial osteotomy (HTO) can change the biological intra-articular microenvironment of osteoarthritic (OA) knees.

Methods

Consecutive patients who underwent medial open-wedge HTO between June 2018 and May 2020 for isolated medial compartment knee OA with varus malalignment were prospectively included. Synovial tissues (STs) and fluids (SFs) were collected from the osteoarthritic knees during initial HTO and prate removal surgeries. Changes in gene expression in STs were investigated using microarray and real-time polymerase chain reaction (PCR). Synovial tissues were also evaluated histologically using synovitis scores and immunofluorescence staining to determine macrophage polarity. Cytokines and chemokines in SFs were analyzed using enzyme-linked immunosorbent assays (ELISA). The mechanism of macrophage polarization was investigated in primary human macrophages from peripheral blood mononuclear cells stimulated with cartilage fragments and SFs. We also evaluated Spearman correlations between knee injury and osteoarthritis outcome scores (KOOS) and macrophage-related gene expression.

Results

Thirty-one patients (19 females, 12 males) with mean age of 61.1 ± 7.5 years were included. Plates were removed 13.2 ± 2.2 months after HTO. Microarray analysis of the three patients identified 477 differentially expressed genes (DEGs) between the samples obtained during HTO and plate removal, and revealed the M1-macrophage-related CCL3, IL1B, IL6, FOS, and JUN gene expressions were downregulated, and the M2-macrophage-related CCL18 gene expression was upregulated. These results indicated a shift in gene profiles from M1 to M2 macrophages and downregulated inflammatory pathways. The KEGG pathway results showed downregulated inflammatory pathways, such as TLR signaling pathway, rheumatoid arthritis, TNF signaling pathway, cytokine-cytokine receptor interaction, and NF-kappa B signaling pathway. Real-time PCR determined that genes expressing pro-inflammatory IL1B and IL6, and M2 macrophage-related IL1RA, IL10, CCL18, and CD206 genes were respectively downregulated and upregulated. Histological findings with hematoxylin and eosin staining showed attenuated synovitis scores and immunofluorescence staining showed a shift from M1 to M2 macrophages. ELISA for SF showed only the concentration of interleukin-1ß decreased after HTO. Cartilage fragments were less in the SF at plate removal than during HTO, and cartilage fragments, not humoral factor of SF, were responsible for M1 macrophage polarization and pro-inflammatory IL1B and IL6 expression in in vitro primary human macrophages. Postoperative KOOS positively correlated with the expression of the M2-related genes CCL18 (r=0.40) and CD206 (r=0.36)

Conclusion

Correction of mechanical realignment altered the biological microenvironment of the knee joint. HTO is a joint preservation procedure that improves mechanical loading that changes inflammatory status in the knee joint and alters synovial macrophage polarization from M1 to M2, to a pro-healing phenotype.