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BYPASSING THE HLA SYSTEM IN ORGAN TRANSPLANTATION:
OPPORTUNITIES TO ACHIEVE UNIVERSAL COMPATIBILITY VIA FULL
SUPPRESSION AND ARTIFICIAL IMMUNOBIOCODING
Davronov Sobirjon Otabek ugli
KIMYO International University in Tashkent, Uzbekistan
Scientific Supervisor:
Sultonova Rukhsora
Abstract
In organ transplantation, the human leukocyte antigen (HLA) system is considered one
of the main immunological barriers. HLA molecules are polymorphic antigens that serve
as primary alloreactive triggers for the immune response. Incompatibility between donor
and recipient HLAs increases the risk of graft rejection. Conventional approaches-such as
compatibility matching, desensitization, or long-term immunosuppression-often provide
limited efficacy. This study analyzes the potential of partially blocking the HLA system
and forming a “Self” signal via artificial biocoding to restrict immune responses in
transplantation.
Key words:
Transplantation; HLA; immunosuppression; desensitization; biocode;
CRISPR/Cas9; tissue compatibility; knockout.
Introduction
Using
gene
editing
technologies
(such
as
CRISPR/Cas9)
and
nanoimmunobiotechnological tools, it is possible to temporarily or permanently suppress
HLA molecule expression, thus advancing the concept of universal donor compatibility.
At the same time, molecular biocoding can potentially conceal the transplanted organ from
immune detection in the recipient. These approaches aim to address donor matching
limitations and significantly reduce rejection rates. According to data from the Global
Observatory on Donation and Transplantation (GODT), more than 150,000
transplantations are performed worldwide each year. Organs and tissues are typically
allocated based on criteria such as urgency, waiting time, histocompatibility, and HLA
sensitization [1]. The introduction of universal approaches in transplantation—
particularly the partial suppression of HLA or its substitution with an artificial biocode-
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offers a novel solution to long-standing challenges. This concept holds the potential to
fundamentally reshape the practice of transplantation.
Main div
In the field of transplantation, particularly in the treatment of cancer and genetic
diseases, modern engraftment methods such as cell therapy and hematopoietic stem cell
transplantation (HSCT) are widely applied. However, these methods are often limited by
alloimmune responses resulting from human leukocyte antigen (HLA) incompatibility.
Successful HSCT requires the elimination of immune responses directed against donor or
recipient HLAs. Suppressing the expression of the HLA-A gene using CRISPR/Cas9
(Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated protein
9) may be a promising approach for increasing HSCT efficacy by expanding the pool of
unrelated donors. Allograft rejection is primarily associated with immune responses
mediated by T and B lymphocytes against HLA molecules. The CRISPR-Cas9 system
functions as a highly accurate gene-editing tool, enabling the cleavage or modification of
targeted genes at the DNA level.
This technology is currently being used to selectively knock out HLA genes-especially
HLA-A, HLA-B, HLA-C, and HLA-DR. When HLA genes are deleted from donor-
induced pluripotent stem cell (iPSC) or embryonic stem cell (ESC) lines using CRISPR,
the resulting tissues and organs are not recognized as “foreign” by recipient T cells. This
offers a path toward realizing the concept of universal donors in transplantation.
According to scientific and clinical data, a 2019 study conducted by Xu H. and
colleagues demonstrated that blocking HLA-A, HLA-B, and CIITA (a major regulator
gene of MHC class II) via CRISPR enabled the generation of pluripotent cell lines
completely lacking HLA class I and II molecules. These cells: were not recognized by T
cells; retained HLA-E expression to reduce susceptibility to NK cells; and ensured long-
term allograft survival without the need for immunosuppression [2].
Furthermore, in 2021, cardiomyocytes fully lacking HLA molecules were developed
by deleting the beta-2 microglobulin (B2M) and CIITA genes using CRISPR. These cells
were accepted without rejection and survived for extended periods in humanized mouse
models [3].
From the perspective of biological safety, complete removal of the HLA system
prevents recognition by T cells, but increases the risk of destruction by NK cells. To
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counter this, molecules that inhibit NK cell activity—such as HLA-E or HLA-G—can be
preserved or artificially expressed, or the expression of immune-suppressive molecules
such as PD-L1, CD47, and CD200 can be upregulated. These strategies not only reduce
immune responses to the graft but also promote the induction of immunological tolerance.
Turning to the results, Kyoto University’s Center for iPS Cell Research and
Application (CiRA) is making consistent progress in developing an iPSC line bank for
transplantation purposes. HLA-homozygous donors were selected, and 27 clinical-grade
iPSC lines were generated from their peripheral or cord blood. Among them, 4 “super
donor” lines were found to be compatible with approximately 40% of the Japanese
population. Current reports indicate that the creation of 140 HLA-A, B, and DR
homozygous lines may provide coverage for up to 90% of the population in Japan [4], and
that CRISPR technology has been used to block genes such as HLA, B2M, and CIITA to
create universal immuno-compatible iPSC lines that are now being tested in heart, kidney,
and liver transplantation models [2].
Although HLA-deficient cells created through CRISPR represent a major achievement
in preventing immune recognition, they are still vulnerable to natural killer (NK) cell
attacks due to the absence of HLA molecules-referred to as the “Missing self”
phenomenon. Therefore, to ensure real compatibility for transplantation, the concept of
an artificial immunobiocode was developed. This approach enables the transplanted cell
to evade immune surveillance while maintaining biological safety.
What is a biocode? It is a set of immunomodulatory molecules that either conceal the
cell’s presence from the immune system or emit a “self” signal. These include: Non-
classical MHC-I molecules such as HLA-E and HLA-G, which bind to inhibitory
receptors (CD94/NKG2A/B) on NK cells and suppress immune activation; Immune
checkpoint molecules such as CD47 and PD-L1, whose expression protects the cell from
phagocytosis and T-cell-mediated attack; Engineered β2-microglobulin–HLA-G fusion
proteins, which act as surface polymers providing immunosuppressive signals [5][6].
In pluripotent stem cells, classical HLA-I molecules were eliminated by knocking out
the B2M gene, and in their place, a β2m-HLA-E single-chain dimer was introduced [7].
This strategy protected the cells from NK cell “missing self” attacks and simultaneously
prevented recognition by T lymphocytes [8].
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Through these approaches: T lymphocytes do not recognize classical HLA molecules,
while NK cells interpret HLA-E/G signals as “Self,” resulting in immunological tolerance,
yet preserving the ability to detect infections or malignant transformations. Clinically,
these immunobiocode technologies are currently being tested in SARs‑T clinical trials,
particularly in iPSC-derived heart, liver, and spinal cord tissues [9].
Such methods have demonstrated the potential to: reduce or even eliminate the need
for immunosuppressive drugs; shorten transplant waiting lists; and ease donor matching
constraints.
Feature
Stability in conventional
transplantation
CRISPR-Based HLA
Block + Biocode
HLA Compatibility
Requirement
Strict matching required
Not required or
significantly reduced
Risk of Allograft Rejection
High
Significantly lower
Immunosuppressive Drug
Use
Long-term, lifelong
Minimal or possibly
eliminated
NK Cell Activation Risk
Low (due to normal HLA
expression)
High (HLA loss triggers
“missing self”)
Clinical Application Status
Established
Experimental (e.g., SARs-
T trials)
Conclusion
Immunological incompatibilities associated with the HLA system remain one of the
primary challenges in transplantation. Practical evidence has shown that editing HLA
genes-particularly HLA-A or the entire HLA class I/II complexes-via CRISPR/Cas9
technology can significantly reduce the immunogenicity of donor cells. Additionally,
artificial “immunobiocode” strategies involving the expression of molecules such as
HLA-G, HLA-E, PD-L1, and CD47 have been shown to protect transplants from both
innate and adaptive immune responses.
These technologies are increasingly recognized as promising approaches for
implementing the concept of universal donors in transplantation, minimizing the need for
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immunosuppressive drugs, and overcoming limitations in donor matching. In the near
future, large-scale clinical studies in this direction are expected to significantly enhance
both the safety and efficacy of transplantation therapies.
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