Discussion: 
Thrombotic microangiopathy is a severe complication seen following HSCT in children, thought to be associated or preceded by graft-versus-host disease (GVHD). We are currently working in the paradigm of a three-hit hypothesis, where patients with underlying genetic factors (Hit 1) that undergo conditioning regimens such as chemotherapy and/or radiation (Hit 2) and experience other potential sources of endothelial injury such as infection, medication or GVHD symptoms (Hit 3) are more likely to manifest the disease (6).
As demonstrated by previous literature, our data show higher incidence of transplant-associated thrombotic microangiopathy (TA-TMA) after allogeneic HSCT when compared to its autologous counterpart (7,8). Prior literature has documented increased risk of both acute and chronic GVHD associated with peripheral blood HSCT (9,10). Additionally, GVHD association with TA-TMA is both well-documented and in agreement with our data (11,12). We also discovered, in concordance with the literature, that GVHD had a higher association with late TMA, defined as TMA that presents 120 days after transplant, compared to early TMA (p = 0.03) (13).
In chronic GVHD, the endothelium can be damaged as a result of cytotoxic T-cell induced injury (14). Other studies have also shown that many of the immunosuppressive agents used to treat GVHD, such as calcineurin inhibitors can cause damage to the endothelium (15,16). Treatments given following transplant such as radiation and chemotherapy are also known to be associated with development of TA-TMA (17–19).
Our data shows a significant correlation between CMV, HHV6, and fungal infections post-transplant and risk of TA-TMA (20). Interestingly, we also found that HHV6 infection increases both risk and mortality in addition to risk of TA-TMA. HHV6 is known to directly infect endothelial cells and inhibit angiogenesis (21). Several studies have noted elevated levels of neutrophil extracellular traps (NETs) in patients with TA-TMA (22). Mechanistically, it is believed that IL-8 is released by the damaged endothelial tissue, recruiting neutrophils that will undergo netosis and form NETs. These NETs, known up-regulators of complement, then induce complement activation on self-cells, leading to TA-TMA. While normally NETs may be cleared, in patients with chronic GVHD undergoing treatment, the endothelium is constantly being attacked (23). In patients additionally infected with HHV6, angiogenesis is inhibited, affecting the body’s ability to repair this damage ( 24).