We evaluated clinicopathological correlates of upper motor neuron (UMN) damage in amyotrophic lateral sclerosis (ALS), and analyzed if the presence of the C9ORF72 repeat expansion was associated with alterations in microglial inflammatory activity.

Microglial pathology was assessed by IHC with 2 different antibodies (CD68, Iba1), myelin loss by Kluver-Barrera staining and myelin basic protein (MBP) IHC, and axonal loss by neurofilament protein (TA51) IHC, performed on 59 autopsy cases of ALS including 9 cases with C9ORF72 repeat expansion.

Microglial pathology as depicted by CD68 and Iba1 was significantly more extensive in the corticospinal tract (CST) of ALS cases with a rapid progression of disease. Cases with C9ORF72 repeat expansion showed more extensive microglial pathology in the medulla and motor cortex which persisted after adjusting for disease duration in a logistic regression model. Higher scores on the clinical UMN scale correlated with increasing microglial pathology in the cervical CST. TDP-43 pathology was more extensive in the motor cortex of cases with rapid progression of disease.

This study demonstrates that microglial pathology in the CST of ALS correlates with disease progression and is linked to severity of UMN deficits.