Role of C-Reactive Protein at Sites of Inflammation and Infection

TLDR: mounting evidence that CRP isoforms have distinct biological properties, with nCRP often exhibiting more anti-inflammatory activities compared to mCRP, and further studies are needed to expand on these emerging findings.

Abstract: C-reactive protein (CRP) is an acute inflammatory protein that increases up to 1000-fold at sites of infection or inflammation. CRP is produced as a homo-pentameric protein, termed native CRP (nCRP), which can irreversibly dissociate at sites of inflammation and infection into five separate monomers, termed monomeric CRP (mCRP). CRP is synthesised primarily in liver hepatocytes but also by smooth muscle cells, macrophages, endothelial cells, lymphocytes and adipocytes. Evidence suggests estrogen in the form of hormone replacement therapy influences CRP levels in the elderly. Having been traditionally utilised as a marker of infection and cardiovascular events, there is now growing evidence that CRP plays important roles in inflammatory processes and host responses to infection including the complement pathway, apoptosis, phagocytosis, nitric oxide (NO) release and the production of cytokines, particularly interleukin-6 and tumour necrosis factor-alpha. Unlike more recent publications, the findings of early work on CRP can seem somewhat unclear and at times conflicting since it was often not specified which particular CRP isoform was measured or utilised in experiments and whether responses attributed to nCRP were in fact possibly due to dissociation into mCRP or lipopolysaccharide contamination. In addition, since antibodies for mCRP are not commercially available, few laboratories are able to conduct studies investigating the mCRP isoform. Despite these issues and the fact most CRP research to date has focussed on vascular disorders, there is mounting evidence that CRP isoforms have distinct biological properties, with nCRP often exhibiting more anti-inflammatory activities compared to mCRP. The nCRP isoform activates the classical complement pathway, induces phagocytosis and promotes apoptosis. On the other hand, mCRP promotes the chemotaxis and recruitment of circulating leukocytes to areas of inflammation and can delay apoptosis. The nCRP and mCRP isoforms work in opposing directions to inhibit and induce NO production respectively. In terms of pro-inflammatory cytokine production, mCRP increases interleukin-8 and monocyte chemoattractant protein-1 production whereas nCRP has no detectable effect on their levels. Further studies are needed to expand on these emerging findings and fully characterise the differential roles each CRP isoform at sites of local inflammation and infection.