Successful organ transplantation requires careful immune
suppression: enough to block the rejection of transplant while permitting host defense
against infectious microbes. Viruses
that are not cleared by our immune systems, are common in healthy people, and can
complicate transplantation include cytomegalovirus (CMV) and Torque
teno virus (TTV), which was first described in 1997 [review]. TTV
is a small (3.8 kb), single-stranded, transfusion transmitted DNA virus,
representative of a highly diverse family of anelloviruses.
The authors examined the influence of immune-suppressive
drugs (e.g., tacrolimus, mycophenolate mofitil, cyclosporine) and the anti-CMV
drug valgancyclovir on chronic, endogenous microbes. From 96 heart or lung transplant recipients
they collected 656 blood samples over time, some up to a year post-transplant, removed
the cells, and identified remaining DNA by sequencing. They found that 0.12% matched viral or
bacterial or fungal sequences. They
validated some ‘hits’ with quantitative PCR.
Control preparations using water or bacteriophage demonstrated no
relevant artifacts or contamination.
They found that treatment with valgancyclovir reduced
herpesviruses, including CMV, but dramatically increased the relative and
absolute levels of anelloviruses, including TTVs (fig. 2, 3, 4). Moreover, those patients who did not reject
their transplants tended to have a greater increase in anelloviruses (Fig. 5A,
shown; rejecting patients plotted in red).
The authors conclude that anellovirus levels might be used to monitor
immune competence. Focosi et al. made a related observation following autologous stem cell transplantation. 
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