A subset of MS brain lesions contain a rim of iron-laden macrophages, detectable with MRI, which correlate with disability. CD163, a scavenger receptor for haemoglobin-haptoglobin complexes, is upregulated in MS brain lesions, but its expression in the clinically relevant spinal cord is unknown.

To assess expression of CD163+ in the MS spinal cord and relate this to iron deposition.

Cervical and lumbar spinal cord sections from 44 progressive MS and 5 control cases were immunostained for CD163, microglia-macrophages (Iba1,CD68), lymphocytes (CD3,CD8,CD20) and myelin (PLP), with lesions staged using CD68. CD163+ cell density was quantified in fields-of-view (FOV) centred on vessels, and 100μm-wide surrounding regions, in non-lesional white matter (NLWM), and in meninges. Iron was detected histochemically using DAB-enhanced Turnbull-Blue staining and iron deposition in cellular compartments (axons, macrophages, astrocytes) was assessed semi-quantitatively. A subset of 20 cases were stained for haptoglobin, which was quantified using an automated pixel-count algorithm.

In controls, CD163+ was confined to vessels and meninges. In MS, CD163+ was seen throughout active lesions, at the rim of mixed active/inactive lesions, and in the NLWM. Iron+ axons were preferentially located in the lateral columns, where their presence related to parenchymal CD163 (ρ=.351,p=0.012). This was maintained correcting for total Iba-1 and CD68 densities. On evaluating MS cases with and without iron+ axons, CD163+ counts were higher in iron+ compared to iron- cases (p=0.004) or control (p=0.038). Meningeal CD163 density did not associate with iron scores, but rather with meningeal lymphocyte inflammation. Haptoglobin coverage was higher in high-CD163+ MS vs low-CD163+ MS cases (p=0.0334)

CD163+ microglia/macrophages are elevated in MS cases exhibiting abnormal iron distribution. CD163+ density relates to haptoglobin deposition in the spinal cord. Together these findings suggest that leakage of blood products across damaged vessels may be an important source of pathological iron in the MS spinal cord.