FISH-analysis of residual cells after sex-mismatched transplantation is mainly based on simultaneous labeling of the centromeres of the X- and Y-chromosomes. Because of possible false positive and false negative results e.g. due to background or hybridization problems, it was necessary to determine the cut off level. Therefore, a total of 26,633 cells from 10 healthy female and 35,783 cells from 11 healthy male were analyzed with the described automated system. The automated analysis showed in the female controls 257 cells with apparent male signal constellation (XY), and the male controls had 142 cells with apparent female signal constellation (XX). To control these automated results we investigated all questionable cells; only 38 out of 257 and 27 out of 142 could be confirmed to be real XY-positive cells or in the male case XX-positive cells. This corresponds to a false positive rate of 0.14% in female and 0.08% in male. An additional random control of 4,841 XX cells in female and of 4,535 XY cells in male showed that there was no further failure of automatic counting.

As the cut off level depends on the amount of analyzed cells, all mentioned female and male cells were listed in spreadsheets with random order and arranged in blocks of 50, 100, 200, 400, 800, 1,500, 2,000, 2,500, 3,000 and 4,000 cells. Subsequently, the mean and standard deviation was assessed for each block. The cut off level was defined as the mean plus twice the standard deviation. The respective cut of levels for each block size were fitted by a trend line enabling the calculation of cut off levels for arbitrary cell numbers up to 4000. Fig. 1 shows the determined/calculated cut off values for female and male cells including trend lines.

In order to determine the false positive rate for trisomy 8 another 15,882 cells from 5 healthy people were analyzed with centromere 8 probes. The mean false positive rate was 1.2%. In the same manner 11,453 cells of 11 healthy controls were investigated using the LSI-probe against the bcr/abl-fusion gene. The mean false positive rate for the bcr/abl-probe was 0.7%. For estimating the cut off level for XX/XY in combination with trisomy 8 (XX or XY+trisomy 8) or bcr/abl (XX or XY+bcr/abl) the 95-quantil with the following formula was used:

Thus, the cut off level was as follows:

• for XX+trisomy 8 and XX+bcr/abl = 0.005%

• for XY+trisomy 8 and XY+bcr/abl = 0.003%.

Two statistical aspects of the present study were further assessed. First, the minimum number of cells to be analyzed was determined in order to achieve a predefined accuracy for the estimated fraction of acceptor cells in the total blood of the patient.

Given the total blood of a transplanted patient consists of N cells of which N_A are acceptor cells, the fraction of acceptor cells in the patient is P_A = N_A/N. A blood sample contains fewer than N cells and the fraction of acceptor cells in the sample p_A can only be an estimate for the true fraction P_A. But how many cells must be analyzed in a blood sample in order to achieve a predefined accuracy? The probability of finding M_A acceptor cells in a blood sample of size M drawn from a total of N blood cells of which N_A are acceptor cells is given by a hypergeometric distribution. However, the sample size M is generally much smaller than the total number of cells N. Thus, the true fraction of acceptor cells P_A can be assumed to be the same before and after the blood sample has been drawn from the patient. The hypergeometric distribution is then well approximated by the binomial distribution . In the present case the exact value of P_A = N_A/N is unknown but is to be estimated by the sample ratio p_A = M_A/M. The mean and standard deviation of the random variable p_A are given by , respectively. For determining the minimal number of cells to be analyzed in a blood sample we use the order of magnitude estimate P_A = 0.01 according to the measurements. The coefficient of variation (equivalent to the relative standard deviation given in %) is then given by the ratio of standard deviation and mean . For the relative standard deviation to be smaller than q the sample size must exceed M* = 99/q². Hence, for q = {100%, 50%, 25%} the required sample sizes must be larger then M* = {99, 396, 1584}, respectively.

Second statistical aspect, the error bounds for the fraction of acceptor cells in the blood sample were calculated with respect to classification errors introduced by the automated cell recognition device. The measured number of acceptor cells may not reflect the real number of acceptor cells in the sample due to measurement errors of the automated device. In order to assess the error rate of the device we classified 10 and 11 samples from healthy females and males, respectively, and obtained histograms for the fraction of misclassified cells. These were similar for females and males and thus pooled in a single distribution as shown in Fig. 2. Using this error distribution we calculated the probability density function for the true number of acceptor cells in the sample as described in the following paragraph. The resulting probability density function indicates that the measurements rather overestimate the number of true acceptor states.

In order to assess the error introduced by the measuring device we calculated the probability distribution for the true number of acceptor cells M_A in the sample given the number of measured acceptor cells M*_A. First note that for a given misclassification probability b the number of correctly measured acceptor cells M*_AA is binomial distributed with total number of cells M_A and probability 1-b. Accordingly, the number of donor cells erroneously measured as acceptor cells M*_AD is binomial distributed with total number of cells M_D and probability b. As shown in Fig. 2 the misclassification probability b itself is β-distributed. Thus, b-averaged binomial probability distributions are obtained by integration according to the β-distribution, i.e.

The probability of measuring M*_A acceptor cells conditional on the fact that M_A true acceptor cells are present in the sample is given by the sum of probabilities consistent with the conditions M*_A = M*_AA + M*_AD and M = M_A + M_D. It reads

in which the product is implied by the fact that the events 'M*_AA out of M_A' and 'M*_AD (= M*_A - M*_AA) out of M_D (= M - M_A)' are independent and must occur at the same time. The probability for the true number of acceptor cells M_A in the sample conditional on M*_A acceptor cells having been measured is given according to Bayes' theorem

From the pooled experimental data the a priory probability for the number of measured acceptor cells was estimated to be a sum of two exponentials. In the present medical context the prior for the number of acceptor cells in the sample f_M(M_A) is best chosen to be uninformative (i.e. constant) in order not to bias the results towards low M_A values, which would conflict with the interest of the patient. Another choice would be to equate . Since is strongly peaked for zero measured acceptor cells the resulting is biased towards low M_A-values, especially for small sample sizes M. This choice could be more interesting for e.g. insurance companies.

Sample plots of the conditional probability distribution are shown in Fig. 3 for the empirical prior. From these plots it becomes clear that the number of acceptor cells is rather overestimated by the measurements since the great majority of cells are donor cells that are occasionally classified as acceptor cells. The opposite case, i.e. acceptor cells being classified as donor cells, is very rare simply because there are only very few acceptor cells.

Overall 454 samples of 58 patients were investigated with X- and Y-chromosome specific probes as described in detail in Tab. 1. 33 patients had a complete chimerism (posttransplant 290 samples) and therefore no residual cancer cells. 25 had a mixed chimerism (47 of 163 posttransplant samples). In order to see the clinical relevance of residual cells, patients with mixed chimerism were divided in two groups, one with less than 4% sexmismatch to the donor (13 patients) and the other with more than or equal 4% sexmismatch (12 patients). Both groups show no correlation with the time since transplantation. Patients with < 4% residual cells were transplanted between 10–55 years (median = 46), patients with ≥ 4% residual cells between 0–66 years (median = 49).

Patients after dose reduced conditioning treatment prior to transplantation (RC) showed a tendency to develop ≥ 4% residual cells whereas in myeloablative repertoire regimes (MRR) patients trend to develop < 4% residual cells. In detail 54% of patients with ≥ 4% residual cells underwent MRR and 38% RC conditioning. In contrast, 25% of patients with ≥ 4% residual cells underwent MRR and 67% RC conditioning. The survival rate of all three groups (complete chimerism, < 4% residual cells and ≥ 4%) is shown as a Kaplan-Meier-plot in Fig. 4. For a detailed compilation of causes of death see in Table 1.

In 12 patients cytogenetic disease-markers were detected before transplantation (bcr/abl-fusion (n = 9); trisomy 8 (n = 3)). For these cases a simultaneous hybridization of the centromeres X and Y together with the bcr/abl- or centromere 8-probe was performed. As shown above it is possible to decrease the cut off level for acceptor cells by targeting gonosomes and tumour specific genome alterations in a single hybridization. 55 samples of twelve patients were investigated. In nine of these samples residual cells were found in a range of 0.1–3.3%. In two of those nine patients (cases 3, 14) the detected sexmismatch cells were not disease-marker positive, whereas in the other seven patients (cases 29, 31, 35, 36, 37, 41, 45) disease-marker positive residual cells were detected (in total 25 cells). Two specimen of disease-marker negative and disease-marker positive residual cells are shown in Fig. 5.

In these remaining groups with small numbers of patients the disease-marker gave no additional information. The amount of disease-marker-positive or negative residual cells showed no correlation with clinical outcome like relapse or death. Table 2 shows the course of 9 patients with residual cells with known disease-marker.

Chimerism analysis after sex-mismatched bone marrow or peripheral blood stem-cell transplantation is an important diagnostic component to monitor transplantation and minimal residual disease and DLI 151617. The FISH technique progresses in importance but demands high personnel skills and costs. An automatic chimerism analysis system could be the solution for that dilemma and was evaluated here. As advantage of automatic analysis using Metafer turned out, that the picture and the coordinates of each cell are memorized. Via basing points it is possible to relocate each cell for further investigation. Moreover, it is possible to analyze huge amounts of cells and to detect small subpopulations of residual cells. Automatic analysis correlates linearly with manual analysis (R² = 0.985) 18. This permits to compare automatic and manual chimerism analysis. Because of the small fraction of the targeted cells in the whole population one should analyze in future studies more than 1600 cells for a reasonable precision (rel. standard deviation of 25%) as described in the statistical part.

Comparing FISH and PCR in chimerism analysis it was shown that the results are in concordance 7. The sensitivity of PCR is between 3–5% and allows only semiquantitative analysis 419 whereas FISH is more sensitive (1%) and is absolutely quantitative 20. Because of the different sensitivities it is recommended to use just one method 21, in patients with complete chimerism the method with the highest sensitivity should be used for early detection of residual cells 1722.

For cut off levels in XY-FISH-analysis it could be shown that they depend on the sum of evaluated cells per sample (e.g. 3,000 counted cells: cut off 1.2% and 0.6% respectively – see Fig. 3. Trakhtenbrot and coworkers 23 described a very alike cut off level in female cells.

In women with sons, male cells could be found 27 24 or 38 years 25 postpartum in blood samples. The authors showed that up to 40.000 male cells could be transplanted in a normal PBSCT by female donors. In case male cells were found in patients after transplantation this would be incorrectly interpreted as residual cells. Therefore, listing of female donors with sons is recommended in anamnesis and should be considered in chimerism analysis. Unfortunately, this information was not available for the investigated patients.

In the present study in two samples residual cells were arranged in conglomerates (patient 36, in sample 20 and 26 month after transplantation). The histological origin of these cells was not investigated. Potentially these cells resembled endothelial cells that derived from injury of the endothelium. To prevent contamination with endothelial cells samples from the third aspiration of a single venous puncture is recommended for the cytogenetic analysis. The false positive rate of 1.2% for trisomy 8 we determined was in concordance with Jenkins et al. 26 and Cuneo et al. 27. The bcr/abl-false positive rate of 0.7% was identical with Amiel et al. 28, Van den Berg et al. 29 and Mühlmann et al. 30. However, overall a big variation can be found in the literature concering false positive rate of bcr/abl which is given between 2–10% 2031323334. Possible reasons for these differences could be: 1) different tissue samples (bone marrow vs. peripheral blood), 2) different cell cycle stage (G1, G2) or 3) different chromatin structure in healthy and moribund cells 35 and 4) different probes. With 95-quantil the cut off level for simultaneous hybridization of gonosomes and disease-markers were estimated and represent 0.005% in XX+trisomy 8/XX+bcr/abl and 0.003% in XY+trisomy 8/XY+bcr/abl. This allows detecting one disease-marker positive residual cell in 20.000 analyzed cells which was claimed already in 1994 36. PCR as alternative diagnostic method does not have this high sensitivity.

33 patients out of 58 had a complete chimerism, 13 patients residual cells < 4% and 12 patients residual cells > 4%. As expected the detection of residual cells > 4% correlated with relapse as described in literature 3738. 66.7% out of patient group > 4% residual cell died because of relapse. Median survival from detection of residual cells and relapse was 6 month and is identical with the data published by Uzunel et al. 39. Other studies could not find a correlation between mixed chimerism and relapse 404142. To what extend mixed chimerism gives evidence about relapse is discussed controversially. In here presented data the occurrence of residual cells was not a marker for relapse. One reason might be the retrospective analysis of patients in this work.

In 12 patients a simultaneous hybridization of gonosomes and disease-markers was applied. 7 patients had disease-marker positive residual cells. But the study showed also that disease-marker positive and disease-marker negative residual cells can be verified within a sample. The detection of disease-marker positive residual cells had no impact on relapse or survival. In contrast Führer et al. 14 could detect disease-marker positive residual cells before relapse. Thiele et al. 13 also arranged a simultaneous hybridization of gonosomes and disease-markers and assumed that cells carrying the disease-marker represent the source for later relapse.

In 3 samples only disease-marker negative residual cells were found. They might represent 1) healthy (benign) leucocytes, 2) precursor tumor cells which do not yet carry the disease-marker, 3) false negative disease-marker positive cells, 4) endothelial cells or 5) cells from female donors with sons.

Peripheral blood samples of 21 clinically healthy male (11) and female (10) between 6 and 67 years were studied as controls.

A total of 28 female and 30 male patients were analyzed retrospectively after sex-mismatched stem cell transplantation which were performed between 1995 and 2006 at the University Medical Centre Jena. As shown in Table 1, there were 24 acute myloid leukemia (AML), 16 chronic myeloid leukemia (CML), 5 acute lymphatic leukemia (ALL), 4 multiple myeloma (MM), 3 severe aplastic anemia (SAA), 2 myelodysplastic syndrome (MDS), and 1 patient each with Non-Hodgkin lymphoma (NHL), chronic lymphatic leukemia (CLL), lymphoma (Lym) and osteopetrosis (OP). Conditioning regimens were dose reduced in 30 patients or myeloablative in 23 patients 444546 54 patients underwent peripheral blood stem-cell transplantation (PBCST) and the remaining 4 bone marrow transplantation (BMT). The median age of the transplanted patients was 46 years (2–89 years). 12 patients showed cytogenetic disease-marker in their malignant cells, i.e. a bcr/abl-fusion in nine and a trisomy 8 in three patients. Overall, 19 patients died, either due to relapse (n = 9), a graft-versus-host-disease (n = 3) or an infection (n = 4). In 3 patients the reason of death remained unclear.

Standard techniques were used to cultivate leukocytes out of venous blood, prepare chromosome-preparations 43, and to perform interphase FISH analysis 44. Commercially available probes (Abbott, Wiesbaden, Germany) for LSI-ES bcr/abl, centromere 8, X and Y were applied.

For automated analysis we used an Axioplan 2 Imaging microscope (Carl Zeiss Jena, Germany) equiped with CCD-camera CV-M1, 1280 × 1024 pixel (Jai Glostrup, Denmark) and a motorized stage with 8 slide positions (Märzhäuser, Wetzlar, Germany). All components were connected to a personal computer (Dell, Langen, Germany) running the Metafer/MetaCyte-Software from MetaSystems (Altlussheim, Germany).

The evaluation procedure of FISH-slides was as followed: 8 slides were automatically scanned over night and the amount of residual cells was registered. Cells which did not have the characteristic signal combination for XX and XY were excluded. All detected potential residual cells were visually controlled by microscope and each valid cell was further examined wether the residual cell carried a disease-marker or not. The system allowed repositioning of all residual cells in order to visually control the group of interest.

Table 3 shows the parameters used for automated scanning.