P-122: Increased mTORC2 pathway activation in lymph nodes of iMCD-TAFRO

Background Idiopathic multicentric Castleman disease (iMCD) is a rare and life-threatening hematologic disorder involving polyclonal lymphoproliferation and organ dysfunction due to excessive cytokine production, including interleukin-6 (IL-6). Data demonstrate that IL-6 inhibition is effective in 34-50% of patients. mTOR, which functions through mTORC1 and mTORC2, is a recently-discovered therapeutic target. The mTOR inhibitor sirolimus, which preferentially inhibits mTORC1, has led to sustained remission in a small cohort of anti-IL-6 refractory iMCD patients with thrombocytopenia, anasarca, fever, renal dysfunction, and organomegaly (TAFRO). However, sirolimus has not shown uniform effect, potentially due to its limited mTORC2 inhibition. Method To investigate mTORC2 activation in iMCD, we quantified the mTORC2 effector protein pNDRG1 by immunohistochemistry of lymph node tissue from six iMCD-TAFRO and eight iMCD patients who do not meet TAFRO criteria (iMCD-not-otherwise-specified; iMCD-NOS). Eight metastasis-free sentinel lymph nodes were selected as normal controls (sentinels). Tissue from six ALPS lymph node samples was selected as a comparator group. Results First, we evaluated mTORC2 activation in iMCD-TAFRO and iMCD-NOS. In iMCD-TAFRO, pNDRG1 expression was significantly elevated in the interfollicular space (P = 0.005), germinal centers (P = 0.002), and mantle zones (P = 0.007) relative to sentinels. Positive pNDRG1 staining was significantly increased in the interfollicular space (P = 0.005) of iMCD-NOS lymph nodes relative to sentinels and there was a no difference in the germinal centers (P = 0.59) and the mantle zones (P = 0.30). Next, we compared pNDRG1 expression in iMCD-TAFRO and iMCD-NOS to ALPS, a disease characterized by aberrant mTOR activation that is effectively treated with sirolimus. While ALPS patients consistently respond to sirolimus, data from the ACCELERATE registry show that 6/11 (54.5%) iMCD cases treated with sirolimus did not achieve a response. We therefore hypothesized that mTORC2 may be more highly elevated in iMCD, suggesting a potential mechanism of resistance to sirolimus. Our results revealed significantly increased pNDRG1 staining in iMCD-TAFRO germinal centers relative to ALPS (P = 0.02) and non-significantly increased staining in the interfollicular space (P = 0.18) and mantle zones (P = 0.11). There were no differences in pNDRG1 expression between iMCD-NOS and ALPS in any region. Notably, the strongly positive pNDRG1 cells had spindle-shaped morphology resembling stromal cells. This result contrasts with the pS6-positive cells in iMCD that have been shown to represent monocytes, plasma cells, and as-yet-undefined cells with myeloid-appearing morphology. Conclusion These results suggest increased mTORC2 activity in iMCD and that duel mTORC1/mTORC2 inhibitors may be a rational therapeutic approach. We also observed potential differences in mTORC2 activation between iMCD-TAFRO and iMCD-NOS, although this should be validated.