From Serology to Sequencing: How HLA Typing Has Evolved Over Five Decades

The methods used to determine a patient's HLA type have changed dramatically since the early days of transplantation. Each advance has revealed new layers of complexity.

When the first successful kidney transplants were performed in the 1950s and 1960s, the tools for characterizing HLA molecules were crude by modern standards. Donors and recipients were matched using serological methods that could distinguish only broad groups of HLA types. Today, next-generation sequencing can resolve individual HLA alleles at the nucleotide level, revealing a degree of diversity that early researchers could not have imagined.

This evolution in typing technology has fundamentally changed our understanding of transplant compatibility, and it continues to reshape how we think about matching.

The serological era

Early HLA typing relied on serological methods: mixing a patient's cells with panels of antisera (antibodies from previously sensitized individuals) and observing which reactions occurred. If the cells reacted with a particular antiserum, the patient was assigned the corresponding HLA type.

Serological typing could identify broad antigen groups (such as HLA-A2 or HLA-B27) but could not distinguish between subtypes within those groups. This was adequate for the level of matching that was clinically feasible at the time, but it missed significant variation that we now know affects transplant outcomes.

Molecular typing

Beginning in the 1990s, DNA-based typing methods began to replace serology. These methods examine the gene sequences encoding HLA molecules rather than the molecules themselves, providing much higher resolution.

PCR-SSP and PCR-SSO (sequence-specific primers and sequence-specific oligonucleotide probes) were early molecular methods that could distinguish between alleles that serology grouped together.

Sequence-based typing (SBT) directly reads the DNA sequence of HLA genes, providing allele-level resolution. This became the standard for high-resolution typing in both organ and stem cell transplantation.

Next-generation sequencing (NGS) is now increasingly used for HLA typing, offering even higher throughput and the ability to resolve ambiguities that affected earlier methods. NGS can determine complete, phased HLA gene sequences in a single assay.

What higher resolution revealed

Each advance in typing resolution has revealed new layers of HLA diversity that affect transplant outcomes. Antigens that were considered single, uniform types by serology turned out to encompass extensive underlying allelic variation, sometimes with clinically significant consequences.

HLA-A2 is the clearest illustration of this. Serology treats it as a single antigen group. DNA sequencing has since identified more than 1800 distinct A2 alleles, most differing by only one or a few amino acids. That scale of hidden diversity is exactly what makes the serological era look crude in retrospect: two donor-recipient pairs could both be reported as "A2 matched" while actually carrying different underlying alleles, some of which fall in the peptide-binding groove and affect which peptides the molecule presents and how the immune system responds to it. Transplants matched at the serological level but mismatched at the allele level can have different outcomes than true allele-level matches.

The data opportunity

The progressive increase in typing resolution has created both a challenge and an opportunity. The challenge is that higher resolution generates more data and more complexity, making simple mismatch counting even less adequate as a risk assessment tool.

The opportunity is that this molecular-level data is exactly what computational models need to identify the specific structural features that drive rejection risk. The information is there in the typing data. What has been missing is the analytical framework to extract its clinical significance. That is the framework Immunomatics is building.