Real-time sequence-specific primer polymerase chain reaction amplification of HLA class II alleles: a novel approach to analyze microchimerism

Naturally acquired microchimerism: implications for transplantation outcome and novel methodologies for detection

Microchimerism represents a condition where one individual harbors genetically distinct cell populations, and the chimeric population constitutes <1% of the total number of cells. The most common natural source of microchimerism is pregnancy. The reciprocal cell exchange between a mother and her child often leads to the stable engraftment of hematopoietic and non-hematopoietic stem cells in both parties. Interaction between cells from the mother and those from the child may result in maternal immune cells becoming sensitized to inherited paternal alloantigens of the child, which are not expressed by the mother herself. Vice versa, immune cells of the child may become sensitized toward the non-inherited maternal alloantigens of the mother. The extent of microchimerism, its anatomical location, and the sensitivity of the techniques used for detecting its presence collectively determine whether microchimerism can be detected in an individual. In this review, we focus on the clinical consequences of microchimerism in solid organ and hematopoietic stem cell transplantation, and propose concepts derived from data of epidemiologic studies. Next, we elaborate on the latest molecular methodology, including digital PCR, for determining in a reliable and sensitive way the extent of microchimerism. For the first time, tools have become available to isolate viable chimeric cells from a host background, so that the challenges of establishing the biologic mechanisms and function of these cells may finally be tackled.

HLA-targeted cell sorting of microchimeric cells opens the way to phenotypical and functional characterization

Microchimerism refers to the presence of less than 1% of non-host cells in a person. Our group developed a reliable method for separating viable microchimeric cells from the host environment. Optimal separation of microchimeric cells at proportions as low as 0.01% could be established with two monoclonal antibodies directed against different HLA antigens, one targeting the microchimeric cells and the other the host cells. Purity of separated cell populations was validated by HLA-allele-specific and Y-chromosome directed real-time qPCR assays. The methodology was used successfully to separate microchimeric maternal cells from child umbilical cord mononuclear cells after pregnancy. Cell sorting with HLA monoclonal antibodies targeting allelic differences enables reliable microchimeric cell detection and separation in blood specimens. With this approach, maximal enrichment of potentially viable microchimeric cells from a background cell population is reached, which opens the way to phenotypical and functional characterization of microchimeric cells.

HLA-targeted flow cytometric sorting of blood cells allows separation of pure and viable microchimeric cell populations

Microchimerism is defined by the presence of low levels of nonhost cells in a person. We developed a reliable method for separating viable microchimeric cells from the host environment. For flow cytometric cell sorting, HLA antigens were targeted with human monoclonal HLA antibodies (mAbs). Optimal separation of microchimeric cells (present at a proportion as low as 0.01% in artificial mixtures) was obtained with 2 different HLA mAbs, one targeting the chimeric cells and the other the background cells. To verify purity of separated cell populations, flow-sorted fractions of 1000 cells were processed for DNA analysis by HLA-allele-specific and Y-chromosome-directed real-time quantitative PCR assays. After sorting, PCR signals of chimeric DNA markers in the positive fractions were significantly enhanced compared with those in the presort samples, and they were similar to those in 100% chimeric control samples. Next, we demonstrate applicability of HLA-targeted FACS sorting after pregnancy by separating chimeric maternal cells from child umbilical cord mononuclear cells. Targeting allelic differences with anti-HLA mAbs with FACS sorting allows maximal enrichment of viable microchimeric cells from a background cell population. The current methodology enables reliable microchimeric cell detection and separation in clinical specimens.

Should microchimerism turn into rejection prophylactics?

Non-self cells can circulate in the body of an individual after any sort of contact with an allogeneic source of cells, thus creating a situation of chimerism that can be transient or prolonged over time. This situation may appear after stem cell transplantation, pregnancy, transfusion or transplantation. Concerning transplantation, many hypotheses have been formulated regarding the existence, persistence and role of these circulating cells in the host. We will review the principal hypotheses that have been formulated for years since the first description of non-self circulating cells in mammals to the utilization of artificially induced chimerism protocols for the achievement of tolerance.

Use of highly sensitive mitochondrial probes to detect microchimerism in xenotransplantation models

Chimerism, defined as the co-existence of cells of different origin within the same organism, has received much attention in hematopoietic cell and organ transplantation because of the strict relationship between its establishment and the induction of specific tolerance. Traditional methods applied for chimerism detection, such as immunohistochemistry, cytogenetics, fluorescent-activated cell sorter analysis, and serological and biochemical testing, are limited by their sensitivity. We have established a highly sensitive molecular approach based on the amplification of the mitochondrial cytochrome B gene and tested its specificity and sensitivity level in six different mammalian species, including human, pig, mouse, rat, sheep and rabbit. Increased sensitivity of detection of specific amplification products was obtained by the non-radioactive Southern blot technique. This novel approach allows the detection of one cell against the background of 1 to 4 x 10(6) xenogenec cells and will be helpful for high-sensitivity analysis of donor cell engraftment after xenotransplantation procedures in these animal models.

Allele-specific quantification of HLA-DQB1 gene expression by real-time reverse transcriptase-polymerase chain reaction

In addition to coding region polymorphism, allele-specific variation in the upstream regulatory region of the HLA-DQB1 gene has been detected. Reporter gene assays and transfection studies have indicated that HLA-DQB1 promoter polymorphism may be of functional significance. The aim of this study was to utilize real-time reverse transcriptase-polymerase chain reaction (RT-PCR) for allele-specific quantification of HLA-DQB1 expression and to analyze cell-specific HLA-DQB1 expression in vivo. For the allele-specific quantification of DQB1 gene products, a real-time RT-PCR set of primer pairs (n=27) and probes (n=5) targeting exon 2 variability was established. The robustness and integrity of the assay system were confirmed by using recombinant DQB1 exon 2 plasmid clones as active exogenous controls. Sensitivity and reproducibility were assessed by serial dilution and allelic mixing analyses. In application to the study of allele-specific expression of DQB1 gene products during cytokine-driven maturation of monocyte-derived dendritic cells, differential patterns of allelic expression in heterozygous individuals were observed for DQB1*0301, compared to DQB1*0501 and DQB1*0602. At maximum, 1.9-fold (*0301/*0501) and 2.5-fold (*0301/*0602) higher induction was seen for DQB*0301. In conclusion, HLA-DQB1 expression can be analyzed by real-time RT-PCR suitable for cell- and allele-specific detection of HLA-DQB1 transcripts in homo- and heterozygous combinations.

Quantification of maternal microchimerism by HLA-specific real-time polymerase chain reaction: Studies of healthy women and women with scleroderma

OBJECTIVE

Microchimerism (Mc), originating from bidirectional fetal-maternal cell traffic during pregnancy, has recently been identified in healthy adults and in patients with scleroderma (systemic sclerosis [SSc]). This study was undertaken to investigate the frequency and quantitative levels of maternal Mc (MMc) in healthy women and women with SSc.

METHODS

HLA-specific primers and fluorogenic probes were used in real-time quantitative polymerase chain reaction assays to detect and quantify MMc by targeting noninherited, nonshared HLA sequences. DNA-based HLA typing was conducted in 67 proband-mother pairs and in all children if the proband was parous. Statistical analysis was limited to 50 proband-mother pairs (including 32 healthy women and 18 women with SSc) in whom MMc could be distinguished from potential fetal Mc.

RESULTS

MMc in peripheral blood mononuclear cells was more frequent among women with SSc (72%) than healthy women (22%) (odds ratio 9.3, P = 0.001). However, levels of MMc, expressed as the genome equivalent of maternal cells per million (gEq/mil), were not significantly different (0-68.6 gEq/mil in SSc patients, 0-54.5 in healthy women). In additional studies, positivity for MMc was demonstrated in a bone marrow aspirate from an SSc patient in whom peripheral blood had been found to be negative for MMc on 4 occasions, and tissue from a subsequent autopsy of this patient had MMc levels of 757 and 1,489 gEq/mil in the lung and heart, respectively.

CONCLUSION

MMc is not uncommon in the peripheral blood of healthy adults, is increased in frequency in patients with SSc, and may be present in bone marrow and disease-affected tissues although absent in the peripheral blood.