HC29: HLA & minor histocompatibility antigens

Allogeneic SCT

There are 2 ways of doing an allogeneic SCT:

• Allogeneic SCT with T-cells present in the cell graft: transplantation of an immune system of a healthy donor with the aim to induce a strong T-cell response against the cancer cells of the patient
  • Mature donor T-cells in the stem cell transplant mediate GVL and GVHD
  • Systemic immunosuppression is required to suppress GVHD
  • Systemic immunosuppression is gradually decreased
• Allogeneic SCT without T-cells present in the cell graft
  • No systemic immunosuppression is required
  • Consists of 2 steps
    • T-cell depletion → no systemic immunosuppression is required
    • DLI is necessary: 3-6 months after allogeneic SCT to induce GVL
      • The patient is in better condition
      • Professional antigen-presenting cells of donor origin
      • Less pathogens
      • Less inflammatory cytokines
  • The T-cell response is weaker → better balance between GVL and GVHD
    • These patients do better as a group → lesser GVHD reaction
      • The body has time to heal damaged tissue from chemotherapy
      • Recipient dendritic cells can be replaced by donor dendritic cells

T-cells in infections

HLA:

HLA molecules are located on chromosome 6. Per chromosome, there are 6 genes which play a role in HLA. Because humans have 2 chromosomes, there are 12 genes in total which play a role. There are 2 types of HLA molecules:

• HLA class I: present intracellular antigens
  • HLA-A
  • HLA-B
  • HLA-C
• HLA class II: present endocytosed antigens
  • HLA-DR
  • HLA-DQ
  • HLA-DP

The HLA groups are located in the peptide binding groove. Only dendritic cells, macrophages and B-cells are capable of HLA-II expression. HLA is highly polymorphic → there are many variants and every different allele has its own name.

Negative selection:

Due to negative selection in the thymus of T-cells which have high affinity for HLA self-complexes, there are no T-cells for processing peptides derived from cellular proteins, otherwise autoimmune reactions would be induced.

T-cells after allogeneic SCT:

Donor T-cells recognize foreign peptide-HLA complexes (allo-antigens). When selecting a matching donor, not all 12 but only 10 genes are taken into account → HLA-DP is usually not taken into account:

• When an unrelated donor (URD) is selected, there usually is a 10/10 match but an HLA-DP mismatch
• When a family donor (sibling donor) is selected, there usually is a 12/12 match with HLA-DP also matching

Therefore, after an unrelated allogeneic SCT, there can be T-cells present in the donor graft which are directed against peptides in mismatched HLA-DP → immune reaction. HLA molecules are major histocompatibility antigens. In case of shared HLA molecules, immune responses against minor histocompatibility antigens can occur.

Minor histocompatibility antigens

A minor histocompatibility antigen (MiHA) can be:

• A polymorphic peptide that differs in amino acid composition between patient and donor
• A polymorphic peptide that is presented on patient cells by HLA surface molecules
  • HLA is matched between patient and donor
• A polymorphic peptide presented by HLA on patient cells that is recognized by donor T-cells after allogeneic SCT

Generation:

MiHAs are produced by differences in single nucleotide polymorphisms (SNPs) between patient and donor. There are >10 million SNPs in the human genome. On average, 10.000 SNP differences are present in a patient transplanted with an HLA matched unrelated donor. A small fraction of the SNP differences encode polymorphic HLA-binding peptides on patient cells that can be recognized by donor T-cells after allogeneic SCT → minor histocompatibility antigens.

Minor allele frequency:

Each SNP occurs in the human population with a minor allele frequency (MAF), for example:

• MAF T = 0,135 → the chance that a T is present on a particular gene position is 13,5%
  • This makes a minor antigen
• MAF C = 0,865 → the chance that a C is present on a particular gene position is 86,5%
  • This makes an allelic variant

With this information, the population frequency can be calculated:

• T/T = 0,135 x 0,135 = 0,02
• C/T = 0,865 x 0,135 = 0,12
• T/C = 0,135 x 0,865 = 0,12
• Total = 0,25 → 25%

The disparity rate is the chance that a patient-donor pair is mismatched for the minor histocompatibility in the right direction → the patient is positive and the donor is negative. If the population frequency of a minor antigen is 0,25, the disparity rate is:

• 0,25 x 0,75 = 0,19 → 19%

Clinical responses:

Minor histocompatibility plays a role in different clinical responses:

• Graft rejection: the patient’s T-cells react to the HLA of cells of donor origin
• GVHD: donor T-cells react to patient cells
• GVL: donor T-cells react to the patient’s leukemic cells

T-cells are the reason for inducing both GVHD and GVL, which can be done by peptides in mismatched HLA-DP and/or minor histocompatibility antigens in matched HLA donors.

For a donor T-cell to engage in GVHD and/or GVL, the donor must not express the minor antigen himself. The patient can be either homozygous or heterozygous expressing this antigen.

Selection of donor T-cells:

Donor T-cells are selected as follows:

1. Donor T-cells are taken either from the bone marrow or peripheric blood
2. Donor T-cells which selectively react with the patient but not with donor cells are selected by measuring the release of IFN-γ in culture supernatants by ELISA
   • Usually immortalized patient and donor EBV-transformed B-cells are used
3. Minor histocompatibility antigens are identified
   • Molecular method
   • Biochemical method
   • Genetic method

T-cell frequencies for minor histocompatibility antigens (MiHA) are higher in patients with GVHD. This can be minimalized if the minor histocompatibility antigens which the T-cells react to are only on hematopoietic stem cells → there is a GVL effect and minimal GVHD on healthy patient tissues. Nevertheless, the majority of MiHA are expressed on both leukemic cells and healthy non-hematopoietic tissues. Healthy non-hematopoietic tissues where minor histocompatibility antigens are often expressed are:

• Gut
• Liver
• Skin
• Lung

Only a minority are hematopoietic restricted minor histocompatibility antigens. This is relevant for immunotherapy → the antigens can be used as an anti-tumor peptide because they don’t attack healthy cells.

Immune responses after allogeneic SCT

In short, relevant factors in immune responses after allogeneic SCT are:

• The number of SNP mismatches encoding minor histocompatibility antigens
• The number of minor histocompatibility antigens that can be recognized by donor T-cells
  • This is dependent on the repertoire of donor T-cell receptor
• The T-cell frequency for each minor histocompatibility antigen
  • GVL is the sum of T-cell frequencies for minor histocompatibility antigens expressed on leukemic cells
  • GVHD is the sum of T-cell frequencies for minor histocompatibility antigens expressed on healthy non-hematopoietic tissues
• The tissue distribution of each minor histocompatibility antigen

If a male patient is transplanted with an HLA matched female sibling donor, the entire Y-chromosome is foreign for the donor derived immune system (this problem doesn’t occur the other way around, because male patients also express X-chromosomes). Male patients transplanted with an HLA matched male unrelated donor have mismatched HLA-DP and minor histocompatibility antigens, but have no anti Y-chromosome reaction. Which patient develops GVHD and which patient does not cannot be predicted yet.

Immunotherapy

Minor histocompatibility antigens can be used for immunotherapy:

• Cellular therapy
  • Adoptive transfer of donor T-cells for hematopoietic-restricted MiHA after allogeneic SCT
  • T-cell receptor gene therapy for in vitro production of MiHA with hematopoietic restricted expression
• Vaccination

Patient or donor DC with hematopoietic-restricted MiHA peptides or mRNA