Overview

The algorithm to follow for donor selection will need to be agreed in consultation between the laboratory and the transplant team and should follow BSHI and EBMT guidelines for the different transplant indications.

Where an allogeneic donor is indicated, historically the patient were HLA typed at medium resolution for HLA class I and II though high resolution HLA class II typing was undertaken for acute leukaemia’s and other urgent transplants to speed up unrelated donor searching if required. Any sibling donors and parents if available, were typed to low resolution for HLA class I and II initially and any that are matched then typed to high resolution class II. The emergence of Next Generation Sequencing (NGS) however now means that many centres to straight to high/allele resolution HLA typing of the patient and relate donors at the start of the donor search process.

For acute leukaemia’s and other urgent transplants, an initial WMDA indicative search may be undertaken ahead of completing the sibling typing. If no matched sibling is found a formal WMDA indicative search is undertaken or the transplant team may decide to proceed straight to a formal unrelated donor search through the national registries depending on clinical urgency. A high-resolution HLA class II type of the patient is required before proceeding to unrelated donor search if not already performed. The WMDA indicative search results are now of such high quality that often, transplant centres select their preferred donors directly from the results of this search rather than waiting for the registry search results.

Where a registry search is undertaken, the registry will initially carry out a national search of all donors. If only 6 or fewer potential unrelated donors are found on the national registries an international search and a cord blood search are automatically requested depending on the agreement between the transplant centre and the national registry.

In most instances, three unrelated donors will be selected from the registry or WMDA searches and samples requested through the registry. The number of donors from whom samples are requested may be increased depending on the assessed likelihood of any units being matched. Choice of donor will be influenced by the registry with some registries having a higher number of deleted donors than others. Registries also differ in the speed of sample provision and in cost of sample provision. The ethnicity of the donor may also provide a clue as to which registries to prefer.

Given a choice of equally HLA matched unrelated donors on the search reports, factors that would be taken into account and the general order in which they will be taken into account are:

• Gender, with male donors preferred over female donors
• Age of donor, with young donors below the age of 40 preferred
• CMV status, with CMV matched preferred over mismatched and
• ABO compatibility with major blood group mismatches avoided if possible

Historically, the requested donors are HLA typed to medium resolution HLA class I and II to start with followed by high resolution class I and II typing of any matched donors. However, the emergence of NGS has meant that the laboratory will often proceed straight to high/allele resolution HLA typing. This may however not be required if the donor is already NGS typed by an accredited registry.

Generally, a fully matched sibling or related donors would be selected over a fully matched adult donor over a cord blood unit. For disease groups where a haplo identical transplant is a clinical option, a haplo sibling or parent donor will be preferred over a 11/12 matched unrelated donor and certainly over an 10/12.

Where a 1 or 2 mismatched transplant is being undertaken, the preferred order of mismatches varies by transplant centre. One option is to prefer mismatches at HLA-C before HLA-A before HLA-B and avoid mismatches at HLA class II if possible. Where a mismatched transplant such as a haplo, a 11/12 or a cord blood unit is being used patient HLA antibody testing will be carried out and any DSA avoided if possible. The HLA antibody testing is repeated at the time of donor work up. Where it is not possible to avoid DSA, a flowcytometric crossmatch can help to stratify the risk. Antibody reduction regimes have been carried out successfully by experienced centres in a few select cases.

Where a CB is being selected, TNC is a more important factor and would be taken into account alongside CD34 count and number of HLA mismatches. The basic standard for selecting cord blood units is:

• Select a CB unit high/allele resolution HLA matched 8/8 at HLA-A, B, C and DRB1. TNC dose should be > 3 x 10⁷/kg at freezing or 2.0-2.5 x 10⁷/kg after thawing
• If selecting a CB unit high/allele resolution HLA matched 5/8, 6/8 or 7/8 at HLA-A, B, C and DRB1, TNC dose should be > 5 x 10⁷/kg. HLA antibody testing must be undertaken to avoid DSA
• Use of CB units < 3 x 10⁷/kg patient weight at freezing and/or 4 or more HLA mismatches at HLA-A, B, C and DRB1 is not recommended
• CD34 counts should be treated with caution due to variations in technique. Aim for 1.5 x 10⁵/kg at freezing or 1 x 10⁵/kg after thawing
• If several units with the same degree of HLA match are available, the one with the highest cell dose should be chosen
• Take into account cell viability and microbiology results

Patients and donors must have their HLA types confirmed before proceeding to transplant. The initial registry type can be considered the first HLA type for unrelated donors with the laboratory HLA type being the confirmatory type (CT). A copy of the confirmatory HLA typing report must be sent to the registry. If there are any discrepancies between the registry type and the CT these must be highlighted.

Wherever possible but especially for the acute leukaemia’s, a backup donor should be identified in case the preferred donor was to fall through.

Patients with rare/unusual HLA Types

Patients with ‘rare’ HLA types can be a challenge to find unrelated donors for. Situations that can give rise to patients with rare HLA type include

• Patients from BAME community
• Presence of a ‘rare’ or low frequency HLA alleles e.g. HLA-DRB1*04:08
• Presence of a low to intermediate HLA type which have several alleles present at a high frequency e.g. HLA-B15 or HLA-DRB1*04
• Presence of a rare HLA-B/Cw association
• Presence of a rare HLA-DR/DQ association
• Homozygosity for some HLA types such as HLA-C and HLA-DR

If a matched related donor is not available, the unrelated donor selection strategy for such patients should take into account the probability of any low to intermediate resolution HLA typed donors being matched at high/allele resolution. The strategy will therefore take into account the most likely regsit5ry from which to call donors or if calling from a predominantly Caucasian registry, the ethnicity of the donors on that registry. There may be a need to call more donors than the three which laboratories typically call in the first instance to increase the probability of finding a match.

Urgent Transplants

The most important factors to take into account when sourcing stem cell donors for a patient are the patient diagnosis and risk factors and so therefore the urgency with which transplantation is needed. Factors to take into account at the time of unrelated donor selection from a seach report include:

• Availability of HLA typing information at HLA-A, -B, -Cw, -DRB1, -DQB1, -DPB1. This is even more important if the patient has a rare or unusual HLA type, unusual associations or if the patient has a HLA type which has several alleles present at a high frequency
• If HLA-DQB1 or HLA-Cw typing is not present, the likelihood of a match based on linkage disequilibrium must be taken into account
• The ethnicity of the donors may provide a clue as the most likely high/allele resolution HLA types
• Presence of molecular HLA type as opposed to serological type can provide some information as to when the donor was typed and therefore how reliable the HLA type may potentially be
• Select donors who have the most likelihood of being a 10/10 or even 12/12 high/allele resolution matched. For intermediate resolution HLA typed donors, this will require looking at the donor HLA string to make sure the patient HLA allele is included in the string
• A choice of a serologically typed donor with a likelihood of being a 10/10 or 12/12 matched may be preferable to a molecularly typed donor who does not carry the patient allele in their HLA string
• Take any null or low expression alleles into account, looking at GvH and HvG directions
• Given a choice of two mismatches, select the allele mismatch over the antigen mismatch
• Given a choice of two allele level mismatches consider if mismatch is permissible:
  • C*03:03 and C*03:04 – the differences are outside the peptide binding groove or α-helices
  • DRB1*14:01 and *14:54 – the differences are outside of exon 2
  • DQB1*03:01 and *03:19 – the differences are outside of exon 2
• Given a choice of equally HLA matched donors to select from:
  • Male donors are preferred over female donors
  • If choice of female donors only, prefer those with no or fewer pregnancies
  • Younger donors below the age of 40 or 45 are preferred over older donors
  • CMV matched donors are preferred over mismatched donors, even if the donor is female
  • Major blood group mismatches are avoided wherever possible
  • If ABO and/or CMV data is not provided on the search report it may be possible to request these from some registries
  • For CMV positive patients, it may be possible to request current/recent results on CMV negative donors last tested more than a year ago
  • Select permissible HLA-DP matched donors if possible
• If using a mismatched donor test patient for HLA antibodies and avoid DSA
• If the patient has ATLL, test donors for HTLV1
• If the patient is HIV infected, test the patient and donors for CCR5
• The reputation of the registry is also a factor to consider. Some registries, such as th NMDP, have a higher number of deleted donors than others. Registries also differ in the speed of sample provision and in cost of sample provision. The DATRI and Saudi Arabian registry for instance are now the quickest to get stem cells from
• Send a copy of the final report to the registry
• Highlight any discrepancies if present and report to the donor registry
• Post-transplant, test for engraftment by chimerism
• If a mismatched donor has been used, test for HLA antibodies post-transplant

Patients with Metabolic Diseases

HSCT is indicated for a number of inborn errors of metabolism, including Hurler syndrome, Scheie syndrome (Mucopolysaccharidosis – MPS IS), Hurler-Scheie syndrome (Mucopolysaccharidosis – MPS IH/S), Hunters syndrome (Mucopolysaccharidosis – MPS II), Mannosidosis (classified as alpha and beta) and Leukodystrophies.

Stem cell transplantation from siblings or fully matched unrelated donors are the standard of care. Mismatched donors are a clinical option though as there is no benefit from a GvL effect, it is not widely used. Factors to take into account at the time of unrelated donor selection from a seach report are similar to those described above for urgent donor selection.

Cord Blood Selection

At Time of Selection

The most important factors are cell count and HLA match. At the time of reviewing the search report:

• Look at total nucleated cell count per kilogram patient weight
• Look at CD34 cell count per kilogram patient weight
• Look to see if high resolution HLA typing results already present at HLA-A, -B, -C and -DR
• Select the largest units in terms of TNC count with the lowest number of HLA mismatches
• HLA matching should be based on high resolution typing at HLA-A, -B, -C and -DR
  • Select an HLA matched 8/8 unit. TNC dose should be > 3 x 10⁷/kg at freezing or 2.0-2.5 x 10⁷/kg after thawing
  • If selecting HLA matched unit 5/8, 6/8 or 7/8 then TNC dose should be > 5 x 10⁷/kg
  • CD34 counts should be treated with caution due to variations in technique. Aim for 1.5 x 10⁵/kg at freezing or 1 x 10⁵/kg after thawing
  • Cord units <= 4/8 are not recommended
• HLA-A and HLA-B mismatches are preferable to HLA-DRB1 mismatches
• Avoid mismatches that the patient has formed HLA antibodies against
• If there are multiple units avoid major blood group mismatches
• If present on the search report, review the volume as a surrogate of freezing method/date
• Check that the cord bank has FACT/NETCORD accreditation
• Request a cord blood report for promising units

Before reserving a unit, request the following if not already known from the unit report:

• From the unit report, determine collection date, processing/volume reduction and cryopreservation method
• Confirm that cord and patient do not have the same date of birth. There is a risk that paediatric patients may have donated cord which would obviously be matched to them
• Is the cord unit red cell replete or has the unit been volume reduced? Prefer volume reduced over red cell replete units
• If the bank is FACT/NETCORD was the unit collected before or after accreditation. An accredited bank could still have ‘unaccredited’ units
• Get gender of cord unit if possible and prefer male donors over female
• Get ethnicity of donor if possible
• Was the TNC determined pre or post volume reduction?
• Review colony forming unit (CFU) viability data
• Does the cord bank have post thaw viability data for this unit? Calculate the TNC when viability is taken into account
• Check microbiology results including of mother and cord blood unit, especially CMV
• Check mother’s medical history if available
• Take non-inherited maternal antigens (NIMAs) into account. Mismatched CB transplants with NIMA matches have been shown to have better outcomes, in terms of the time to blood count recovery and overall patient survival
• Any post donation medical history of the cord blood donor?
• Check availability of DNA sample
• Depending on the cord bank and availability of DNA, request local high-resolution at HLA-A, -B, -Cw, -DRB1, -DQB1 and -DPB1 typing of any promising units
• If choice of multiple units what is known about the speed of provision of units of the cord bank?
• What is the cost of units for both reservation and for provision?
• For transplants undertaken treat a genetic condition, check for the presence of the mutation/SNP in the cord unit

At Time of Reservation

Upon reserving a cord blood unit, request the following if not already:

• Confirmatory typing plus any extended or high/allele resolution typing required
• Some cord banks will also conduct HLA typing on the maternal sample at this stage
• The lab will provide the patient high/allele resolution HLA typing information is to the cord blood bank
• The cord blood bank will send an extended cord report to the lab. If upon review of the additional information the transplant centre wishes to proceed, a cord blood reservation form is completed
• Cord units are reserved for several weeks, typically 6 weeks but this can be extended
• The transplant unit will receive confirmation of the reservation from the cord bank together with instructions on how to organise delivery and the thaw instructions
• Upon reservation, the cord blood bank will carry out standard infectious marker tests, including HIV, HCV, HTLV and CMV, on any stored maternal samples to create the compete ‘donor file’
• The transplant unit can request additional infectious markers be tested for at this stage
• The cord blood bank will send an extended cord blood report, the ‘donor file’, to the transplant centre for review

If proceeding to transplant with the cord blood unit, the following steps are taken:

• The transplant centre sends a ‘Request to Ship’ form to the cord blood bank detailing shipping and transplant dates
• At this stage, some cord blood banks conduct chimerism testing on a line segment attached to the bang, to confirm the identity of the unit just before shipping
• The transplant centre receives the cord unit and sends a line segment to the laboratory for bio-identity confirmatory HLA typing on a line segment
• The transplant centre completes the transplant
• If bio-identity confirmatory HLA typing has not been performed on a line segment, the used bag is sent to the laboratory for urgent HLA typing using residual cells from the bag to confirm identity and HLA type of the transplanted unit

Auto HSCT

Auto HSCT is the standard of care for HSCT for a number of conditions, such as Hodgkin’s Lymphoma and Multiple Myeloma. Patients undergoing Auto HSCT do not have the problem of rejection or GvHD but it is nevertheless important that the H&I laboratory undertake HLA typing before treatment starts as such patients may need an urgent HLA matched blood products as part of their treatment should they have or develop HLA antibodies. In some conditions, such as multiple myeloma with specific molecular cytogenetic abnormalities, consolidative allogeneic HSCT following initial auto HSCT has been shown to extend progression-free survival (PFS) as well as overall survival (OS). Auto followed by Allo HSCT is another reason why it is important that the H&I laboratory undertake HLA typing for patients undergoing Auto HSCT.

HLA typing of potential sibling donors may also be undertaken for diseases where allo transplantation is a clinical option even if auto HSCT is the standard of care.