Immigration check for neutrophils in RA lining: laminin alpha5 low expression regions act as exit points

A correlation exists between the absence of alpha5-laminin and transit checkpoint fenestrations in vascular basement membranes. We hypothesized that similar laminin alpha5 low expression regions might exist in synovial lining, which, although lacking basement membrane, contains all basement membrane components in its interstitial matrix.

Laminin alpha4 and alpha5 chains and lactoferrin were stained using immunofluorescence and cathepsin G and neutrophil elastase using immunoperoxidase. Quantitative real-time polymerase chain reaction (qRT-PCR) was used to measure laminin alpha4 and alpha5 mRNA copy numbers in cultured synovial fibroblasts, without/with tumour necrosis factor-alpha (TNFalpha) and interleukin-1beta (IL-1beta).

Laminin alpha4 and alpha5 chains were found in the intercellular matrix in synovial lining samples of trauma and revision total hip replacements. Laminin alpha5 was weaker in osteoarthritis (OA) and rheumatoid arthritis (RA), and RA synovial lining also contained local low expression areas. Double staining disclosed convergence of lactoferrin-degranulating neutrophils towards these laminin alpha5 low expression regions. In cultured OA synovial fibroblasts, laminin alpha5 mRNA decreased (p < 0.05) at 1 ng/mL TNFalpha and was not found at all in cultured resting or cytokine-stimulated RA fibroblasts. Degranulation of cathepsin G and neutrophil elastase was seen in neutrophils passing through blood vessels or synovial lining.

Migrating neutrophils in RA seem to use laminin alpha5 chain low expression regions to exit synovial tissue to enter synovial fluid. Transmigrating neutrophils remodel the intercellular matrix by releasing their proteolytic granular contents to enhance these low expression checkpoints and/or to produce chemotactic stimuli. In RA fibroblasts this is facilitated by cytokine-mediated down-regulation or lack of laminin alpha5 synthesis.