This study was performed to investigate the effect of conditioning in vivo and graft manipulation ex vivo on the elimination of adaptive immunological memory in the setting of ASCT [
15]. This was tested using two T-cell-dependent vaccine antigens, the neoantigen rabies (HDCV) [
11] and the recall antigen TT, and specific antibody production and antigen-stimulated T cell proliferation as read-outs. Before ASCT, nearly all evaluable children (JIA/SLE cohort) and all adult patients (MS cohort) mounted a primary humoral response to rabies and a memory response to TT and all patients mounted a cellular response to TT. After ASCT, striking differences between the pediatric and adult patient groups were observed: 100% of JIA/SLE patients, but only 33% of the MS patients reached the humoral responder status after the third tetanus revaccination (see Fig.
3). Based on avidity testing, about 56% of the responders showed a memory response. All JIA/SLE patients and 63% of the MS patients were able to respond to the rabies vaccination at 6 months after ASCT. In all but two the response to the rabies revaccination had characteristics of a primary response, i.e., neither an isotype switch from IgM to IgG nor avidity maturation had occured. These data clearly show that in most cases, the immunological memory to a T-cell-dependent neoantigen was eradicated after conditioning. A vaccination with the T-cell-dependent recall antigen TT was given shortly before bone marrow harvest. Therefore, the response to TT after ASCT not only reflects the effect of the conditioning, but also the potential transfer of adaptive immunity by the graft. The humoral response to TT was severely suppressed for a prolonged period of time in MS patients; it was present in 60% in JIA/SLE patients after the first revaccination and restored rapidly after repeated vaccinations (see Fig.
3). The deficient anti-TT antibody production in adult patients may in part be explained by a higher IgG anti-TT titer remaining present after ASCT and before revaccination, compatible with an ongoing production of specific IgG antibodies by radioresistent plasma cells [
28‐
30]. This phenomenon has also been observed in healthy multivaccinated adults [
22] and after ASCT for malignant diseases in adults [
31,
32]. Consistent with this is the finding that preexisting oligoclonal IgG-bands remain present for a long time in the cerebrospinal fluid in four out of five of the included MS patients after ASCT [
4]. This was also observed by others [
33,
34]. The meaning of the latter finding for the course of the pathological process is unclear. Two other factors possibly relevant for the obvious difference in responses to a recall antigen in adults versus children are the more intense conditioning and the more rigorous T cell depletion of the graft in the MS cohort. The biological effective dose (BED) of the TBI can be calculated [
35] to be 15 Gy in adults versus 5.6 Gy in children. The former equals the intensity of myeloablative pretreatment protocols for SCT in hematological malignancies. The T cell depletion of the autograft in MS patients was at least five times more rigorous than in children, and probably prohibited any transfer of T cell memory. In fact, similar findings were previously reported by our group and others [
36‐
41], after myeloablative conditioning, in vivo T cell depletion and allogeneic or autologous SCT. The transfer of a T-cell-dependent humoral memory response after immunoablative conditioning and SCT with a not rigorously T cell depleted graft (leaving 1.0 × 10
4 CD3+ cells/kg body weight) was demonstrated by chance in the present study by the particular JIA case who was vaccinated with rabies before the (second) bone marrow harvest: this child mounted a secondary response after rabies revaccination post-ASCT (see Fig.
6). In conclusion, the results of the present study indicate that immunoablative conditioning may be sufficient to eliminate immunological memory generated against a neoantigen given after graft harvest and before conditioning. On the other hand, as illustrated by the secondary humoral response to TT in 60% of the children after ASCT, the same transplant procedure including moderately stringent T cell depletion of the graft was insufficient to eliminate immunological memory for a recall antigen boosted before graft harvest. The therapeutical effect on the disease was quite different between children and adults: 16 out of 17 evaluable children were cured or remitted of disease progression, whereas only 3 out of 10 evaluable adults improved or had stable disease during follow-up. Whether the difference in kinetics of T-cell-mediated immunological recovery after a transient suppression, i.e., more rapid in children than in adults, influenced the post-ASCT course of the autoimmune disease cannot be substantiated in this evaluative study. Apart from the obvious differences in etiopathology of JIA/SLE and MS other transplant-related factors may have contributed to the disappointing therapeutical effect in MS patients. For instance, the graft of children contained a higher number of T cells than the graft of adults, due to differences in depletion techniques. Although it is generally thought that the recurrence of disease post-ASCT either reflects the presence of autoagressive cells in the stem cell graft or the persistence of these cells in the host, evidence is increasing that further depletion of T cells is not the way to improve the outcome of ASCT and can possibly even lead to more relapses, as seen in our MS patients; the lack of therapeutical effect may be the result of the depletion of regulatory T cells [
3,
42,
43]. Autoantigen-specific regulatory T cells have so far not been studied in AID; the reappearance of nonspecific CD25+FoxP3+ T cells after ASCT has been described by de Kleer et al. [
44], but their exact role in controlling JIA is yet unknown. In MS, such studies have not been performed. The severe and prolonged B and T cell immune dysfunction following the intensive (rather myeloablative) pretreatment of MS patients in this study, as shown by their slow immune recovery for vaccine antigens following ASCT, may have been inappropriate for a regulated and equilibrated nonautoimmune restoration of their adaptive immunity. Further study on autoantigen-specific regulatory T cells before and after ASCT in T-cell-dependent AID may throw more light onto the mechanism behind dysregulated immunity in these patients.