Though still widely performed and considered the gold standard for diagnosis in GCA, TAB has limited sensitivity rendering many false negative results mostly due to non-contiguous inflammation or a large-vessel phenotype. It is widely recognized that even with TAB of sufficient length and appropriate surgical technique, only about 30% of TAB are positive.8 In untreated GCA, the risk of ischemic complications including irreversible vision loss looms large. As such, the treatment paradigm involves early initiation of high dose glucocorticoids, which themselves are associated with significant potential morbidity in this elderly population. As newer biologic therapies, such as tocilizumab, are introduced for GCA, the onus remains on the clinician to make the accurate diagnosis despite imperfect diagnostic tools. A test, like pERM intensity score, which could enhance the sensitivity of routine histopathology and capture the clinically relevant population of patients with biopsy negative GCA could be of tremendous diagnostic utility. The negative predictive value of high intensity score was substantial at 84%. Such a test could potentially spare patients with negative biopsy from prolonged exposure to glucocorticoids or other immunosuppressive therapy. A strength of this study was confining our analyses to those with negative TAB as prospective clinical data allowed for distinction between those with biopsy negative GCA and those without GCA. Conversely, other promising biomarkers, such as vascular endothelial growth factor (VEGF), that have been looked at in TAB specimens, were only identified in positive biopsies, limiting clinical value.9
The current study was limited to subjects with negative TAB. We did not include those with positive TAB in this study as the primary interest was the utility of pERM intensity score in enhancing diagnostic yield of TAB. We did stain and score nine positive TAB, all of which demonstrated high intensity staining, as expected. As we previously reported, the lymphocytes which infiltrate the artery stain intensely for pERM, making independent analysis of the surrounding vasculature difficult. We also chose to exclude positive TAB in this study as when scoring the negative biopsies, there was true blinding as to clinical diagnosis which is obviously not possible in a positive TAB. One could hypothesize that the inflammatory milieu in early GCA that precedes lymphocyte recruitment causes ROCK activation. Thus, activated ROCK in the vasculature perhaps facilitates the subsequent panarteritis that is pathognomonic of biopsy positivity,
The TAB specimens and clinical data from this study came from the TABUL study, which aimed to compare temporal artery ultrasound to TAB.7 Using clinical judgement as the reference diagnosis, TAB was reported to have a sensitivity of 39% with 100% specificity compared to ultrasound with sensitivity of 54% and lower specificity of 81%. Variability in sonographer technique and interpretation is often cited as a concern with this user-dependent imaging modality; there was only moderate inter-rater agreement between sonographers who had all undergone a standardized training program. Thus, while there is an understandable desire to use non-invasive imaging techniques to make a GCA diagnosis, at this time ultrasound cannot completely supplant TAB. The use of pERM intensity scoring on a negative biopsy enhanced the sensitivity of TAB for GCA to 86% 95%CI: 70,95..
The findings in the current study serve to validate our previous findings suggesting ROCK activity is increased in the vasculature of subjects with GCA regardless of biopsy status compared to controls. The strong inter-rater reliability between the pathologists scoring the pERM staining further bolster these results. Though we used pERM as a surrogate of ROCK activation in this study, we did also look at the non-phosphorylated ERM protein. There was no difference in the ERM intensity staining in GCA subjects or controls suggesting the pERM differences demonstrated were caused by disease-related activation of the ROCK pathway.
The mechanistic overlap between aberrant ROCK activation and pathophysiologic vascular changes in GCA provide biologic plausibility for these findings. ROCKs play a critical role in maintaining cellular homeostasis and aiding in injury response in the vasculature while simultaneously being necessary for the propagation of the Th17 immune response. In GCA, the simultaneous activation of both the Th1 and the Th17 pathways have long been implicated in disease pathogenesis.10 The Th17 pathway gives rise to IL-17A expression, which locally enhances the pro-inflammatory milieu and has pleiotropic effects on the surrounding vascular endothelium and smooth muscle. These cell types parallel the areas of high intensity pERM staining and may explain why we can detect pERM staining without an inflammatory infiltrate in those with biopsy negative GCA compared to controls.
While there is immediate diagnostic potential in these current findings if a commercially available stain can improve sensitivity of TAB, there may also be therapeutic implications. Inhibitors of the ROCK pathway have been shown both in vitro and in vivo to reverse T cell dysfunction and downregulate Th17 related cytokines in autoimmune diseases like systemic lupus erythematosus and psoriasis.11,12 ROCK inhibition likely helps to ameliorate autoimmunity by restoring the balance of regulatory Tcells (Tregs). Thus, the ROCK pathway may be a viable therapeutic target in GCA. Furthermore, a proposed mechanism for the efficacy of tocilizumab in GCA is that the blockade of IL-6 receptors restores and enhances the suppressive function of Tregs13 It is conceivable that ROCK inhibition could offer cytokine modulation with similar reestablishment of Treg balance in GCA without the simultaneous immunosuppression seen with IL-6 receptor blockade.
In conclusion, the pERM intensity score can enhance diagnosis of GCA in those with a negative TAB. This underscores the importance of the ROCK pathway in GCA pathogenesis and may offer a promising future therapeutic target.