The literature data suggest that chimerism analyses are routinely performed for the surveillance of engraftment. In recent years, these studies have become the basis for therapeutic intervention [11, 12]. The polymerase chain reaction of short tandem repeats (PCR-STR) method used in our study is a reliable tool, standardized in chimerism laboratory and well described in literature as the routine method for the assessment of post-transplant chimerism [13, 14]. The previous literature data suggested that serial mixed chimerism analysis in patients with acute leukemias at the short time intervals by PCR provides a reliable and rapid screening method for early detection of relapse . Based on its limited sensitivity to detect minor cell population of about 1%, monitoring of chimerism in the whole blood is not suitable to serve as the minimal residual disease (MRD) marker. For the assessment of MRD, other techniques should be used .
Relapse after transplantation is a major cause of treatment failure in pediatric patients with ALL. Barrios M et al.  presented results which indicate that sequential determination of chimerism allows to predict relapse and death after HSCT for acute leukaemia. Patients with increasing mixed chimerism (IMC) showed a significantly higher (p < 0.001) rate of relapse (93.1%) and death (89.7%) in comparison to both those with complete chimerism (CC) (29.9% relapse, 44.1% dead) or decreasing MC (11.1% relapse, 44.4% dead). Relapse was found in 39.8% of analysed patients. The correlation between donor chimerism status and disease relapse after alloHSCT was investigated by Jiang Y et al . 21.6% patients had recurrent disease. In the analysed group relapse was observed in 9% of patients who showed increasing recipient chimerism, although on day +14, they were complete chimeras. Five-year OS and 5-year event-free survival (EFS) were 62.07 ± 4.37% and 56.17 ± 4.38% respectively, for overall cohort of patients with ALL in the Pachon C et al. study . In our study OS (83%) and EFS (84%) were higher than described in literature . Three of our patients are alive with stable mixed chimerism (10% donor; 35% donor and 90% donor, respectively), which is in line with the observation of Levrat E. et al with very long-term stability of MC in patients with haematologic malignancies .
Lassaletta A et al.  presented results of the analysis of chimerism by the day 30 after peripheral blood progenitor cell transplantation. 27/39 patients showed CC by day 30 after HSCT, but median time to achieve CC was 15 days (range 8–750). In 15.4% patients, CC was never reached. On the day +7, median 61% of our patients presented very early complete donor chimerism, whereas on the day +14 median 90% patients had complete donor chimerism. Finally, 48/56 (86%) of our patients achieved complete donor chimerism.
Comparable to other studies, in our cohort no correlation between early donor chimerism and source of stem cell transplanted was found, but the number of transplanted CD34+ cells had a significant impact on patients’ chimerism status [21, 22]. We observed that matched unrelated and male donor were connected with high level donor chimerism on day +7 and +14, similar to other report .
A relatively low-dose ATG is effective in acute GvHD prophylaxis, leading to promising survival rates in matched transplants . Our results indicate that ATG is also effective, but connected to mixed chimerism in very early period after HSCT. In patients who received ATG as GvHD prophylaxis, engraftment with complete donor chimerism was observed later than in patients without ATG (day +21 vs +14 respectively).
Conditioning regimen (myeloablative) did not affect early chimerism status, which is compatible with other reports [25, 27].
Some studies report, that the status of CC by day +30 was significantly related to the development of chronic GvHD. Patients who presented CC by day +30 had 80.8 +8%, probability of developing chronic GvHD, whereas the probability of patients with MC by day +30 was 48 +17% (P = 0.04) . Mossalam G et al.  observed, that low donor chimerism in patients was connected with a reduced risk of chronic GvHD. Jaksch M et al.  found a significantly higher risk (p = 0.005) for developing aGvHD grades II-IV in patients with completed donor CD4+ T-cell chimerism day 7 after SCT together with patients who increased 50% or more in donor CD4+ T cells between days 7 and 10 after SCT. We did not observe correlation between early donor chimerism and aGvHD or cGvHD,perhaps due to the GvHD prophylaxis. The patients who developed cGvHD, presented on day +7 (p = 0.05) donor chimerism above 80%. To confirm this finding, the studies should be continued on a larger group of patients.
Horn B et al.  describe long term follow up of children with acute leukemia with early mixed chimerism-based posttransplant immunotherapy. Children receiving posttransplant immunotherapy achieved similar outcome to patients achieving full donor chimerism spontaneously. Rettinger E et al. added that the immunotherapy in the patients with mixed chimerism improves survival in childhood ALL and does not increase risk of acute GvHD . In our cohort a successful therapeutic intervention was undertaken twice on the basis of chimerism measurements in the early posttransplant period (day +21).