P234 Typing challenges associated with mixed chimerism

Mixed chimerism refers to the presence of DNA from 2 different individuals in a patient’s peripheral blood and can present interesting challenges when HLA typing. We present 2 examples of patients with mixed chimerism and the steps taken to confirm their typing. The first case is a 27 yo with relapsed AML. HLA typing was performed on peripheral blood using RSSO but was uninterpretable as the test controls performed but the software was unable to assign typing. Testing was repeated with SSP which also resulted in no solution at initial software analysis. Further inquiry with the clinical team revealed that the patient had previously undergone a haploidentical transplant at another centre. Upon manual review of the SSP typing results, 3 haplotypes could be identified. Typing results from the original transplant centre confirmed the suspected haplotypes. A buccal sample from the patient was typed and matched the patient’s pre-transplant typing.The second case is a 7mo patient with SCID undergoing HLA typing for stem cell transplant. Initial typing on DNA isolated from peripheral blood and performed by SSP resulted in 3 different antigens at the B locus and the C locus. Further testing was performed by RSSO and the result was uninterpretable. Typing was also completed on the mother and father to look for a possible donor and to assist with haplotype analysis. The family study showed that the patient demonstrated as expected 1 HLA-B and C antigen from his father, but also demonstrated both B antigens and both C antigens from his mother. This picture is consistent with maternal fetal chimerism. The other loci in the case did not demonstrate extra antigens due to sharing in the parental haplotypes. HLA typing of patients with mixed chimerism can be difficult. A pertinent history from the clinical program is essential for identifying possible mixed chimeras and will prompt the laboratory team to request a buccal sample. A review of the previous donor’s typing or a family haplotype analysis can also aid in identifying potential chimeras. Our lab had eliminated the use of buccal sample processing for patients with leukopenia after obtaining DNA robots for isolation. However, we have re-instated this method after these cases highlighted the potential need to process DNA from alternate sources.