Despite CBT being practiced for 2 decades, relatively little is known about the infection risk and immune recovery post-transplant. High rates of infection have been reported with single unit CBT18-20
, and use of ATG has been linked to a high risk of viral infections5-9
. Further, there is little data available concerning the infection risk and immune recovery after double unit CBT. Improved understanding of infection risk and immune recovery is critical to the implementation of more effective prophylactic and therapeutic anti-microbial strategies. Moreover, immune recovery is of particular interest not only from the standpoint of infection risk, but also given Parkman and colleagues have demonstrated an association between decreased leukemic relapse and successful recovery of immune responses against Herpes viruses after CBT21
Our study is the first detailed analysis of infection risk after double unit CBT without ATG. We demonstrate that serious opportunistic infections remain a major problem. However, while bacterial and fungal infections were frequent, mortality from such infections was rare. This is likely due to aggressive management of febrile neutropenia, prophylaxis against and coverage of mold infections, and possibly by improved engraftment with double unit grafts. The only risk factor we identified for the development of these early infections was conditioning intensity. Non-myeloablative conditioning was associated with a delayed time to bacterial/ fungal infections, similar to transplantation of other hematopoietic stem cell (HSC) sources22
. Additionally, there were no cases of Pneumocystis jirovecii
pneumonia or Toxoplasmosis gondii
infection. While this success is important, bacterial and fungal infections (including pneumonias) remain a significant burden, especially after myeloablative conditioning. More rapid neutrophil recovery after double unit CBT would be beneficial and will require a larger CB inventory, ex vivo
expansion, or other strategies23
. Currently, the best prophylaxis for bacterial infections is unclear given the emergence of gram negative organisms resistant to quinolones, and the increasing incidence of other antibiotic resistant organisms such as VRE. Clinical care would also be considerably enhanced with the implementation of improved methods to establish the causative organism for clinical pneumonias, and strategies to prevent VRE bacteremia in colonized patients.
Viral infections, by contrast, were more likely to cause or contribute to death. While HSV and VZV infections were effectively prevented by acyclovir as reported with other HSC sources24,25
, CMV infections posed a challenge. Consequently, close monitoring for CMV reactivation is imperative in order to rapidly institute pre-emptive therapy. We found a similar incidence of CMV viremia and disease as previously reported after CBT26-29
indicating that CMV represents a significant risk after double unit CBT despite omission of ATG. Our observation that early CMV infection was not prevented by avoidance of ATG, and had no relationship with GVHD, is consistent with the report of Beck et al29
. Further, while CMV-specific T-cell responses developed as early as day 60, CMV contributed to early deaths in 2 patients. Our center does not routinely prescribe CMV prophylaxis given the risk of myelosuppression from ganciclovir/ valganciclovir and the nephrotoxicity of foscarnet. Thus, while our data support the findings of Cohen et al that naïve CB lymphocytes can relatively rapidly generate antigen specific responses30
, more effective and less toxic CMV prophylaxis and treatment are needed31
. In addition, our CMV CF antigen response is only a semi-quantitative proliferation assay primarily evaluating CD4+ T-cell responses without assessing the cytolytic function of cytotoxic T-cell responses. More sophisticated measures of CMV-specific T-cell responses such as intracellular cytokine-based assays32
would be of interest to more accurately assess CD4+ and CD8+ CMV-specific T-cell recovery, especially comparing CBT with different methods of GVHD prophylaxis.
The incidence of HHV6 viremia at our institution33
was higher than we report in this study, but here only symptomatic patients or those with rapidly rising viral load who received HHV6 therapy were included. High rates of HHV6 viremia have been observed after CBT34-36
, and the uncertainty of its significance is extremely problematic. While one of our patients developed graft failure possibly related to HHV6 viremia as reported by Chevallier36
, and another died of HHV6 encephalitis37,38
, some asymptomatic viremia resolved without therapy. A better understanding of the clinical course of HHV6 is required to differentiate patients that may derive benefit from potentially toxic anti-viral therapy, and those who may be safely observed.
Our incidences of infections with adenovirus4,7
appear lower than previously reported after CBT, possibly due to our omission of ATG from the conditioning. Notably, in our study both cases of adenovirus enteritis, and all cases of EBV, occurred exclusively in the context of systemic GVHD therapy or corticosteroid administration for another indication. Monitoring for EBV reactivation is thus warranted in any CBT recipient receiving systemic immunosuppressive treatment for GVHD or corticosteroids for another reason to facilitate immediate pre-emptive therapy. While EBV viremia resolved with rituximab in 4 patients, the single patient with EBV lymphoma required third-party EBV-specific cytotoxic T-lymphocytes17
. Both patients with adenovirus gut disease died, and, although the primary cause of death was GVHD, these viral infections contributed to death.
While we were unable to identify any significant predisposing factors to development of the first viral infection other than recipient CMV sero-positivity, it is notable that mortality from all infections, including viruses, was limited to the first 4 months post-transplant, suggesting the development of immune reconstitution sufficient to prevent serious infections even in patients with ongoing GVHD. We examined ALC and CD3+4+ T-cell counts as simple measures of immune recovery given CD3+4+ T-cell recovery is protective against opportunistic infections15,39
. We observed more rapid CD3+4+ T-cell recovery than two early reports of ATG-based single unit CBT40,41
, and two more recent CBT series of single or double unit CBT in 32 patients each8,9
. Komanduri et al reported a median 6 month CD3+4+ T-cell recovery of approximately 100 cells/microL after single unit ATG-based CBT8
as compared with 356 cells/microL in our study. Brown et al found a similarly low median 6 month CD3+4+ T-cell count of approximately 100 cells/microL in adult reduced intensity ATG-based double unit CBT recipients9
. Further, the PHA responses in our patients (median 80% LLN by day 120) exceed those previously reported in a single unit ATG-based CBT series (median <50% LLN by 6 months)40,41
. While such retrospective comparisons to published series is limited by the confounding factors of potentially differing patient populations and conditioning regimens, differences in immune recovery in the absence of ATG are suggested. Interestingly, while omission of ATG did not prevent early CMV, a benefit was manifest in the risk of later infections such as EBV or adenovirus or late CMV given patients not exposed to GVHD therapy or corticosteroids for another indication were not at risk for EBV, adeno-viral gut disease, or late CMV at all. By contrast, patients with GVHD or corticosteroid therapy had a significant infection risk, with 92% of late viral infections after day 120 occurring in this context.
In summary, the mortality risk from bacterial and fungal infections has been significantly abrogated by anti-microbial agents to prevent and treat infection, and possibly by the use of double unit grafts. While the bacterial/ fungal infection risk of myeloablative conditioning is increased compared with non-myeloablative conditioning, such infections were unlikely to be lethal. Viral infections, especially early CMV infections, remain challenging. However, we show a reduced incidence of EBV and adenovirus infections compared to published literature, possibly due to our omission of ATG. Without ATG we have achieved a high rate of sustained engraftment. Our incidences of GVHD are not prohibitive, although it should be acknowledged that a significant minority of patients will develop severe GVHD. Further, the mortality risk from infection was restricted to the first 4 months post-transplant, with the reduced risk coincident with recovery of ALC, CD3+4+ and CD3+8+ T-cells, and PHA responses. Nonetheless, further improvement is indicated. This will require more rapid neutrophil recovery and more effective prevention of GVHD, ideally without the profoundly T-cell depleting effects of ATG. Improved anti-viral agents and investigation of cellular therapy to augment anti-viral immunity should also be a major priority in CBT. The investigation of more sophisticated measures of immune recovery such as T-cell receptor rearrangement excision circles 8,9
, T-cell differentiation8
and T-cell repertoire41
will also be of great interest in ATG-free double unit CBT recipients, although the absence of lethal infections after 4 months post-transplant is the most powerful surrogate marker of immune recovery4
. These findings support double unit CBT without ATG, and will act as a baseline for the investigation of new strategies to further prevent infection and augment immune recovery.