Investigating the role of galectins in models of acute and chronic inflammation

Abstract

The role of endogenous Galectin-1 (Gal-1) in acute inflammation has been poorly investigated. I therefore performed the carrageenan-induced paw oedema model in wild type and Gal-1-/- mice. Upon sub-plantar injection of carrageenan, Gal-1-/- mice displayed a similar first phase of oedema (≤24h) to wild-type mice, however a much less pronounced second phase (48-96h) was evident in this genotype. This reduced inflammation was associated with a reduced cellular infiltrate and lower expression of inflammatory genes in the paw. Analysis of galectin expression at both mRNA and protein level revealed high expression of Gal-1 in wild-type paws during resolution (≥48h), with elevated expression of Galectin-9 (Gal-9) also observed. In Gal-1-/- mice, Gal-9 protein expression, mainly in recruited immune cells, was remarkably high. Administration of stable Gal-1 to wild-type mice completely ablated the first phase of oedema but was ineffective when administered therapeutically at the 24h time-point. Conversely Gal-9 administration did not alter the first phase of oedema but significantly reduced the second phase when administered therapeutically. This suggests anti-inflammatory actions for both proteins in this model albeit at different phases of the inflammatory response. Collectively, these data indicate that the absence of endogenous Gal-1 results in an abrogated response during the second phase of the oedema reaction.

In more chronic settings, Gal-1 has gained much interest as a potential therapeutic target given the tonic anti-inflammatory actions displayed by this protein. Evidence with regards to its exogenous application in both gene and protein therapy in a model of collagen-induced arthritis (CIA) were shown to inhibit disease progression and severity (Rabinovich et al., 1999). In the current study I queried if I could unveil similar functions for endogenous Gal-1, by performing the CIA in Gal-1 null mice.

Gal-1 null and C57BL/6 (WT) mice received an initial immunisation of chicken type II collagen (CII) in complete Freund’s adjuvant (FA) followed by a booster on day 21, which consisted of CII in incomplete FA. Animals were monitored daily for signs of arthritis from day 14 onwards. Histological analysis was performed with basic H&E to monitor synovitis and pannus formation. Safranin O staining was also performed to monitor cartilage degradation. Lymph nodes (LN) were collected at selected time-points from arthritic animals of both genotypes and ex vivo stimulation of LN cells with CII was performed to measure T cell proliferation and production of cytokines. Anti-CII antibody response was also measured in serum of arthritic animals of both genotypes

Over the course of the CIA, there was a distinct disease penetrance, with ~70% of Gal-1 null mice developing arthritis compared to ~50% in WT animals. The mean day of disease onset was also accelerated in the Gal-1 null (day 25.25 ± 1.10 post-immunization) compared to WT (28.27 ± 2.30) mice. A significantly (*P<0.05) more severe arthritis was also apparent with the maximum clinical score in Gal-1 null mice significantly elevated compared to WT. Histological analysis of joints revealed marked cartilage and bone erosion with pannus formation and loss of joint structure, however when mice of similar clinical scores were compared, there were no differences in the degree of synovitis and joint destruction between genotypes. Significantly elevated levels of all three anti-CII immunoglobulin isotypes (IgG1, IgG2a and IgG2b) were found in the serum collected at day 42 of Gal-1 null animals when compared to WT. Lastly, T cell responses following specific ex-vivo stimulation with CII revealed a greater degree of proliferation in T cells of Gal-1 null mice compared to WT, which was also associated with anincreased production of IL-17.

Collectively these data indicate - for the first time - that endogenous Gal-1 serves as a tonic inhibitory factor in the development of arthritis affecting, in part, disease severity. I have also highlighted the importance of endogenous Gal-1 in regulating T cell responses during experimental arthritis. Strategies aiming at potentiating/mimicking Gal-1 would provide novel approaches to the clinical management of human rheumatoid arthritis.