Reducing immune rejection related mortalities of cellular therapies using “stealth” technologies

Author: Jun Yan Tan, Editor: Dr Chih Wei Teng

Introduction: Current issues with allogeneic therapies

Cellular therapies have recently gained significant traction and use in the treatment of severe injuries and chronic diseases such as cancer. However, these treatments carry significant complications that are life-threatening to patients. Risks include adverse effects from chemoradiation conditioning, infection from immune suppression, organ damage and Graft-versus-host disease (GVHD).1 GVHD is considered one of the more troublesome complications with the number of patients growing 3.5% annually from the increased number of allogeneic HSC transplants being performed. It currently affects 15 thousand people globally each year.2,3

GVHD is a condition following the transplantation or engraftment of allogeneic cells where donor (graft) T cells attack immunocompromised tissue in recipients (host). The condition affects between 35-50% of patients receiving a first allogeneic HSC transplant and records a mortality rate of 18% for acute GVHD and 28% for chronic GVHD.2,4,5 (Table 1)

Table 1: Incidence & mortality of acute and chronic GVHD from first allogeneic HSCT, extracted from GlobalData 23/05/22.2

16MM*Incident Cases (N)Incident Cases (N)Mortality (N)Mortality (N)Rate of Incidence (%)Rate of Incidence (%)Mortality Rate (%)Mortality Rate (%)
*16MM represents combined results from 16 countries;
**projected figures based on historical data.

A contributing factor to the mortality rate are the difficulties of matching donors and recipients according to their human leukocyte antigens (HLAs). Better matches for donor recipients are critical as this results in a lower rate of treatment-related mortality, for example, when leukaemia patients receive a bone marrow transplant.6

Despite best efforts to reduce this incidence through high HLA matching, even ideal matches (HLA-identical siblings) result in acute GVHD at a rate of 21% across Europe and the U.S..7 The reality for many patients seeking HSC transplantations is that few will have an HLA-matched sibling (25% chance for siblings with the same parents)8 and will have to rely on donor populations. Moreover, the chances of finding suitable unrelated donors (URD) can be significantly reduced for minority races due to poor representation in the donor population.9 Figure 2 shows a low chance of an 8/8 HLA match, uncommon haplotypes of HLAs in the African American population being as low as 5%,10 resulting in a higher proportion of treatment-related mortalities.

Figure 2. 8/8 HLA match rate by race (pre (initial search results) and post (after donor HLA typing) 2009 registry.10
Indeterminate in “pre” typing columns is defined as pseudo-patients with potential unrelated donor match at 8/8, but further typing is required.
Indeterminate in “post” typing columns is defined as pseudo-patients without a specified 8/8 match, where
     additional unrelated donors remain untested.
WH, White; HIS, Hispanic; API, Asian/Pacific Islander; AFA, African American/Black.

Current prophylaxis (preventative treatment) and treatments for immune rejection

Current protocols and prophylaxis for stem cell transplantation require extensive preparation against immune rejection. There are varying levels of conditioning regimens depending on the degree of HLA matching.9 This serves two purposes: to decrease the number of cancer cells and destroy native HSCs, making immunologic space and reducing graft rejection.11 Myeloablative conditioning administers doses of total body irradiation or alkylating agents to effectively wipe out the host’s bone marrow stem cells.12 Reduced intensity conditioning regimens are primarily used to suppress the host’s immune system and can also work by taking advantage of graft-versus-malignancy effects.12, 13

Additionally, patients will be treated with immunosuppressants such as methotrexate and corticosteroids to prevent immune rejection, even in cases of autologous stem cell transplants.14,15 These potent drugs already have many adverse effects, including increased risk of future cancer, loss of appetite, nausea, vomiting, hair loss, bladder bleeding, infertility, allergic reactions, pulmonary fibrosis, sexual dysfunction, increased chance of infection and vision changes. When treatments consist of a cocktail of chemotherapy drugs (e,g. cyclophosphamide, alemtuzumab, fludarabine and melphalan) and immunosuppressants taken together, it results in significantly reduced quality of life for patients.

Despite precautionary measures, many patients receiving allogeneic or autologous HSC transplantation still get GVHD. Treatment options following the onset of GVHD are limited.16 Currently, corticosteroids are the first in-line treatments and are effective in less than 50% of patients, and second-line treatments have poor outcomes (50% survival at six months and 35% at 12 months). 17,18

Because of these factors, the rate of incidence and mortality of GVHD is expected to remain consistent.2 This represents a significant issue with current treatments and subsequently, the need for more preventative measures to reduce the burden of immune rejection of cellular therapies. Additionally, lowering the risk of immune rejection may reduce the reliance on total body irradiation and immunosuppressants, subsequently reducing the adverse effects of treatment.

Stealth Strategies: emerging options to evade immune rejection

An emerging approach uses “stealth” techniques to avoid host immune cell recognition, potentially eliminating immune rejection and reducing subsequent rejection or associated side effects.

One novel “stealth” method involves the use of an alloimmune defence receptor (ADR), which selectively depletes cytotoxic lymphocytes (that target the injected therapy cells) whilst excluding the resting lymphocytes. Preclinical studies have demonstrated these effects in Chimeric antigen receptor (CAR)-T cells and CAR-NK (natural killer) cells, where allogeneic CAR-T/NK cells derived from induced pluripotent stem cells (iPSC) were able to resist host immune rejection whilst still retaining a similar level of anti-tumour activity .19,20

Precision Biosciences are testing another method of evading immune rejection with their stealth allogeneic CAR-T, PBCAR19B, undergoing phase I clinical trials. It uses a stealth vector carrying a short hairpin RNA (shRNA) which effectively reduces immune rejection by cytotoxic T cells through the suppression of beta-2 microglobulin expression (a class I HLA) on the injected CAR-T cells. This shRNA modification results in the treatment cells evading the host’s adaptive immune rejection (T cells) while using an HLA gene to target the inhibitory receptor of NK cells.21 The phase I clinical study for this therapy is expected to have a primary completion date of September 30th, 2022.22

A similar strategy can also be applied to stem cells through gene editing; iPSCs could be made hypoimmunogenic for a universal source of stem cells without the risk of immune rejection. One such method explored is the suppression of HLAs (to avoid recognition) and simultaneous CD47 over-expression (inhibiting phagocytosis). This has been demonstrated in an experiment where mouse- and human-induced pluripotent stem cells (iPSCs) in both undifferentiated and differentiated forms were able to evade immune rejection in major histocompatibility complex (MHC) mismatched allogeneic recipients.23

Implications for cellular therapies

These stealth technologies could potentially support next-generation off-the-shelf allogeneic cellular products that could avoid host rejection whilst providing the same or improved therapeutic benefit.19 These therapies could reduce treatment-related mortalities whilst also reducing the side effects, improving the patient’s quality of life.

As immune rejection remains the most significant risk factor for cellular therapies, reducing this risk could result in more treatments being able to pass Phase I clinical trials, make it to market and fulfil unmet needs.24,25 Thereby, stealth therapies may facilitate new therapeutics that would not otherwise be possible.


Despite best efforts to prevent immune rejection, there remains a significant risk of treatment-related mortality; stemming from complications such as GVHD. With the introduction of an effective stealth system, morbidity and mortality can be significantly reduced whilst lowering the requirements of HLA matching and the need for immunosuppressive drugs, conditioning therapies and facilitating the development of new cellular therapies for many chronic and incurable diseases. Achieving this milestone is particularly important for minority populations poorly represented in the donor population.


This review has been undertaken by CCRM Australia Clarity’s internship program. CCRM Australia Clarity is a one-stop industry research specialist and provider of actionable intelligence in regenerative medicine and advanced therapies. We help our clients in getting solutions to their research requirements through our syndicated and consulting research services. Our internship program provides work integrated learning experience and supports the professional growth for both postgraduate and higher research students.

CCRM Australia Clarity reports provide in-depth analysis on the topic and discusses drivers, restraints and opportunities available in the market. The service is designed to help CCRM Australia Clarity clients in their decision support system.


Figure 1. Cumulative incidence of Treatment-related mortality following bone marrow transplantation.6

Table 1. Multivariate analysis of relative risks of acute and chronic GVHD for different HLA matching.6

MatchingNo.Relative Risk95% CIP
Grade II-IV acute GVHD*
HLA-identical siblings3,4221
8/8 matched URD5262.442.14 to 2.79< .0001
7/8 class I mismatch2142.652.20 to 3.21< .0001
7/8 DRB1 mismatch352.641.69 to 4.13< .0001
6/8 class I mismatch1282.812.22 to 3.56< .0001
6/8 mixed mismatch (single DRB1 + single class I mismatch)283.272.02 to 5.29< .0001
7/8 DRB1 v 7/8 class INA10.62 to 1.60NS
Chronic GVHD†
HLA-identical sibling3,4501
8/8 matched URD5081.971.71 to 2.26< .0001
7/8 class I mismatch2041.641.32 to 2.05< .0001
7/8 DRB1 mismatch291.430.85 to 2.38NS
6/8 class I mismatch1192.231.69 to 2.95< .0001
6/8 mixed mismatch (single DRB1 + single class I mismstch)221.330.60 to 2.99NS
7/8 DRB1 v 7/8 class INA0.870.5 to 1.5NS
NS, Not significant


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