Virus-specific T-cells: Current and Future Use in Primary Immunodeficiency Disorders

Abstract

Viral infections are common and can be potentially fatal in patients with primary immunodeficiency disorders (PIDD). Since viral susceptibility stems from poor to absent T cell function in most patients with moderate to severe forms of PIDD, adoptive immunotherapy with virus-specific T-cells (VST) has been used as to combat viral infections in the setting of hematopoietic stem cell transplantation in multiple clinical trials. Most trials to date have targeted cytomegalovirus, Epstein-Barr virus, and adenovirus either alone or in combination, though newer trials have expanded the number of targeted pathogens. Use of banked VSTs produced from third-party donors has also been studied as a method of expanding access to this therapy. Here we review the clinical experience with VST therapy for patients with PIDD as well as future potential targets and approaches for use of VSTs to improve clinical outcomes for this specific patient population.

INTRODUCTION

Primary Immunodeficiency Disorders (PIDD) are a growing spectrum of defects in adaptive and innate immunity, ranging from potentially fatal diseases of infancy to subtle abnormalities with onset in later life.(1) Severe combined immunodeficiency (SCID) and other major forms of PIDD are targets for early identification in order to allow definitive therapy such as hematopoietic stem cell transplantation (HSCT) or gene therapy prior to onset of potentially life- threatening infections.(2, 3) In this regard, newborn screening via the TREC assay has been highly successful in enabling early identification and preventative care for this population.(4) Beyond SCID, the diagnosis of many other profound forms of PIDD are an indication for hematopoietic stem cell transplantation.(5, 6) Many studies have demonstrated that the presence of active infections in recipients undergoing transplantation adversely impacts survival.(2, 3, 7) Viral infections are very common in patients with forms of PIDD impacting adaptive immunity, and in spite of the success of antiviral medications, respiratory viruses and herpesviruses remain leading contributors to transplant-associated mortality.(5, 8, 9)

For over 20 years, adoptive immunotherapy with virus-specific T-cells (VSTs) has been utilized predominantly in the setting of HSCT for patients with malignancies.(10–24) Early studies predominantly focused on treatment of cytomegalovirus and Epstein Barr Virus by infusing T- cells derived from stem cell donors recognizing viral epitopes shortly after HSCT, and therefore narrow the time window during which recipients have absent effector T-cell function. VST production methods have been recently reviewed.(25) Briefly, early methods utilized EBV- transformed lymphoblastoid cell lines derived from the donor to generate T-cell products after repeated cultures stimulations over several weeks.(17, 19) Advances in VST generation utilized viral transduction or transfection of antigen presenting cells in order to generate VST products recognizing multiple viruses (Figure 1).(16) Most recently, use of peptide libraries spanning multiple viral antigens has allowed rapid expansion of VST products targeting a wide range of viral targets in 10–12 days.(26) Other methods for generating VSTs using rapid selection technologies have also been successful using either MHC multimers for a given viral epitope, or using cytokine capture technologies with immunomagnetic bead separation.(27–31) These methods have the advantage of rapid VST production (i.e. within 24 hours), as well as the use of a commercially available closed system.

Figure 1:

Methods of producing Virus-specific T-cells from peripheral blood mononuclear cells (PBMC) utilize either cell selection or ex vivo expansion. Selection utilizes either multimer sorting or immunomagnetic column selection to isolate VSTs after stimulation based on expression of interferon-gamma (Ifn-g) or CD154. Ex vivo expansion utilizes either antigen presenting cells (APCs) that have been transduced with viral antigens, or direct stimulation of PBMCs with viral peptide libraries to stimulate expansion of VSTs.

Over the past two decades, more than 500 patients have received VSTs in various phase I-II trials internationally. Though most VST use has been in the setting of HSCT for malignant conditions, a proportion of patients with PIDD have been included in these published trials. Most VST products have been derived from stem cell donors for use post-HSCT, but several studies have demonstrated the utility of administering partially HLA-matched VSTs derived from healthy third-party donors as an “off the shelf” VST therapy.(24, 32–37) Successful antiviral responses following third-party VST therapy has improved over time as methods of donor/recipient matching has improved, with response rates in recent trials ranging from 75–92%. (36–38)

Monoviral VSTs in PIDD

EBV, CMV, and adenovirus are among the most common viral causes of morbidity and mortality in patients with PIDD receiving HSCT, and have been the targets of most monoviral VST trials (Table 1). Many early studies utilized ex vivo expansion to produce monoviral VST products. A large study by Heslop et al treated 114 patients with donor-derived EBV-specific VSTs post HSCT. Within this trial, 14 patients received their HSCT for an underlying PIDD.¹⁴ The majority of the patients in this study were treated prophylactically since a history of an EBV-related malignancy or underlying primary immunodeficiency placed them at high risk for developing EBV-lymphoproliferative disease (EBV-LPD) in the T-cell depleted transplant setting. The remaining patients were treated for active EBV-LDP or lymphoproliferation. Of the PIDD patients with active EBV reactivation at the time of infusion, 3 of 6 had complete virologic response to the EBV-specific T cell therapy. Three patients had partial virologic responses, of whom two are alive and one patient died of progression of T-cell lymphoma. Eight patients with PIDD received the EBV T cells as adjuvant therapy post HSCT including six patients with a history of viremia or EBV-LPD. None of these patients developed EBV reactivation post-infusion.

Table 1:

Demographics and Outcomes of PIDD Patients in VSTs Studies.

Study	PIDD Diagnosis	Infection(s)	VST Specificity	VST method	VST Source	Cell Dose	Outcomes
Bao et al.(65)	Hyper IgM Syndrome	CMV	CMV	Culture, peptide	HSCT donor, peripheral blood	5}105/kg	Persistent viremia
	SCID	CMV	CMV	Culture, peptide	HSCT donor, peripheral blood	5}105/kg	Resolution of viremia
	CID	CMV	CMV	Culture, peptide	HSCT donor, peripheral blood	2.5}105/ kg	Resolution of viremia
Creidy et al.(31)	HLH	Meningo encephalitis; retinitis	AdV	Selection by IFN-y secretion	Haploidentical HSCT donor, leukapheresis	1) 5}104/ kg 2) 4 1.5}104/ kg	Alive, stabilization of retinitis
	SCID	Meningo encephalitis; retinitis	AdV	Selection by IFN-y secretion	Haploidentical HSCT donor, leukapheresis	1)4 5}104/kg 2)4 5}104/kg	Resolution of viremia; stabilization of retinitis
	HLH	Pneumonitis; encephalitis; retinitis; viremia	CMV	Selection by IFN-y secretion	HSCT donor, leukapheresis	1.76}104/k g	Died at Day+3 (alveolar hemorrhage)
	HLH	CMV viremia	CMV	Selection by IFN-y secretion	Haploidentical HSCT donor, leukapheresis	1)4 3}104/kg 2) 4 5.88}104/ kg 3) 5 1.99}105/ kg	Died at Day+96; pulmonary arterial hypertension
	CID	Retinitis	CMV	Selection by IFN-y secretion	HSCT donor, leukapheresis	5}104/kg	Alive but blind
Feuch et al.(27)	HLH	Adenoviremia, diarrhea	AdV	Selection by IFN-y secretion	HSCT donor, peripheral blood	5}104/kg	Clearance of AdV from blood and stool
Leen et al.(16)	SCID	Adenoviremia	CMV, EBV, AdV	Culture, DC and LCL with Ad5f35f- CMVpp65 vector	HSCT donor, peripheral blood	1}108/m2	Clearance of AdV
Leen et al.(15)	SCID	Prophylaxis	EBV, AdV	Culture, LCL with Ad5f35 vector	HSCT donor, peripheral blood	1.35}108/ m2	Alive, no active infections
Gerdemann et al.(13)	HLH	CMV, Adv	CMV, EBV, AdV	Culture, peptide	HSCT donor, peripheral blood	2}107/m2	CR
Papado- polou et al-(12)	LAD	Adenoviremia	CMV, EBV, AdV, HHV6, BK	Culture, peptide	Haploidentical HSCT donor, peripheral blood	2}107/m2	CR
	GATA2	EBV, BK viremia	CMV, EBV, AdV, HHV6, BK	Culture, peptide	HSCT donor, peripheral blood	2}107/m2	CR
	SCID variant	BK viremia; EBV viremia	CMV, EBV, AdV, HHV6, BK	Culture, peptide	HSCT donor, peripheral blood	2}107/m2	CR
	HLH	HHV6, BK viremia	CMV, EBV, AdV, HHV6, BK	Culture, peptide	HSCT donor, peripheral blood	1}107/m2	HHV6: CR; BK: NR; EBV: CR
						1- 2}106/kg/d
Vickers et al.(33)	CID	PTLD (EBV)	EBV	Culture, LCL	Third Party	ose; 4 doses given weekly 1- 2}106/kg/d	CR
	CGD	PTLD (EBV)	EBV	Culture, LCL	Third Party	ose; 4 doses given weekly 1- 2}106/kg/d	PD; died
	CID	PTLD (EBV)	EBV	Culture, LCL	Third Party	ose; 4 doses	PD; died
						given weekly
						2}106/kg/d
						ose; 7 doses
						given weekly; 2
Wynn et al.(40)	CTPS1	Primary CNS Lymphoma	EBV	Culture, LCL	Third Party	additional doses given after	CR
						re-
						emergenc e of EBV
						disease
Heslop et al.(14)	WAS	EBV viremia	EBV	Culture, LCL	HSCT donor, peripheral blood	2}107/m2	CR
	NK defect/	EBV viremia	EBV	Culture, LCL	HSCT donor,	1}108/m2	CR
	SCAEBV				peripheral blood
	SCAEBV	EBV	EBV	Culture, LCL	HSCT donor, peripheral blood	2}107/m2	PR, died of progressive lymphoma
	SCAEBV	EBV-LPD	EBV	Culture, LCL	HSCT donor,	2}107/m2	No further EBV
					peripheral blood		reactivation
	XLP (SLAM	EBV viremia	EBV	Culture, LCL	HSCT donor,	2}107/m2	CR
	mutation)				peripheral blood
	XLP and lymphoma	Prophylaxis	EBV	Culture, LCL	HSCT donor, peripheral blood	2}107/m2	No viremia
		EBV-LPD
	CID, enterocolitis	(EBV viremia	EBV	Culture, LCL	HSCT donor, peripheral blood	2.5}107/m2	No further EBV reactivation
		resolved)
	CID, enterocolitis	Prophylaxis	EBV	Culture, LCL	HSCT donor, peripheral blood	2.5}107/m2	No viremia
		EBV-LPD
	WAS	(EBV viremia	EBV	Culture, LCL	HSCT donor, peripheral blood	2.5}107/m2	No further EBV reactivation
		resolved)
	XLP	Resolved EBV viremia	EBV	Culture, LCL	HSCT donor, peripheral blood	2}107/m2	No further EBV reactivation
	XLP	EBV viremia	EBV	Culture, LCL	HSCT donor,	2}107/m2	PR, Alive and
					peripheral blood		well
	XLP	Resolved EBV	EBV	Culture, LCL	HSCT donor,	2}107/m2	CR
		viremia			peripheral blood
	XLP	EBV-LPD with	EBV	Culture, LCL	HSCT donor,	2}107/m2	PR, Alive and
		viremia			peripheral blood		well
	XLP-like	Resolved EBV viremia	EBV	Culture, LCL	HSCT donor, peripheral blood	2}107/m2	No further EBV reactivation
Doubrovina et al.(24)	HLH	EBV-LPD	EBV	Culture, LCL	HSCT donor, peripheral blood	1×106/kg × 3 doses	PD; died
	XLP	EBV-LPD	EBV	Culture, LCL	Third Party	1×106/kg × 3 doses	CR
	ALPS	EBV-LPD	EBV	Culture, LCL	HSCT donor, peripheral blood	1×106/kg	NE; died
Uhlin et al.(41)	SCID	CMV	CMV	Selection by HLA pentamer (CMV- pp65/NLV)	Third-party (maternal, pretransplant)	24.6×104/ kg	PR, died
Naik et al.(39)	IL7RA-SCID	Prophylaxis	CMV, EBV, AdV	Culture, DC and LCL with Ad5f35f- CMVpp65 vector	HSCT donor, umbilical cord	1.5×107/m2	No viremia
	IL2RG-SCID	Prophylaxis	CMV, EBV, AdV	Culture, DC and LCL with Ad5f35f- CMVpp65 vector	HSCT donor, umbilical cord	2.5×107/m2	No viremia
	IL2RG-SCID	Prophylaxis	CMV, EBV, AdV	Culture, DC and LCL with Ad5f35f- CMVpp65 vector	HSCT donor, umbilical cord	1×107/m2	No viremia
	IL2RG-SCID	Prophylaxis	CMV, EBV, AdV	Culture, DC and LCL with Ad5f35f- CMVpp65 vector	HSCT donor	1×107/m2	No viremia
	SCID	Prophylaxis	AdV	Culture, DC and LCL with Ad5f35 vector	HSCT donor	1×107/m2	No viremia
	MHC II deficiency	CMV viremia, pneumonitis	CMV, EBV, AdV	Culture, DC and LCL with Ad5f35f- CMVpp65 vector	HSCT donor	1×107/m2	CR
	WAS	Prophylaxis	CMV, EBV, AdV	Culture, DC and LCL with Ad5f35f- CMVpp65 vector	HSCT donor	1×107/m2	No viremia
	CID, enterocolitis	Prophylaxis	EBV	Culture, peptide	HSCT donor	2.5×107/m2	No viremia
	ADA-SCID SCID SCID	EBV-LPD CMV viremia CMV viremia, pneumonitis	CMV, EBV, AdV CMV, EBV, AdV CMV, EBV, AdV	Culture Culture Culture	Third party (pre-HSCT) HSCT donor HSCT donor (1st dose), Third party (2nd dose	5×106/m2 5×106/m2/ dose;2 doses 1×107/m2 (1st dose), 2×107/m2 (second dose)	NR, died from EBVLPD PR, alive and well NR; died; refractory CMV and disseminated BCG
	MHC II Deficiency	Prophylaxis	CMV, EBV, AdV	Culture	HSCT donor	5×106/m2	CMV: CR
	HLH (STXBP2)	CMV, EBV viremia	CMV, EBV, AdV	Culture	HSCT donor	1×107/m2/ dose × 2 doses	CMV: CR; EBV: CR
	WAS	Prophylaxis	CMV, EBV, AdV	Culture	HSCT donor	2×107/m2	No viremia
	IL-10R deficiency	Adenoviremia, pneumonitis	CMV, EBV, AdV	Culture	Third Party	2×107/m2	AdV: CR; CMV: CR
	HLH	EBV viremia	EBV	Culture	Third Party	2×106/kg/d ose × 3 doses	EBV: PR; Died; PTLD
	CD4 lymphopenia	CMV viremia	CMV	Multimer selection	HSCT donor	2×106/kg	CMV: PR; died of fungal pneumonia
Miller et al.(42)	SCID	Adenoviremia	CMV, EBV, AdV	Culture	Third party × 2 (pre-HSCT)	2×107/m2	Adv: CR
Punwani et al.(66)	MALT1	CMV	CMV	Culture	HSCT donor	NA	CMV: CR
Withers et al.(38)	Hyper IgM Syndrome	CMV	CMV	Culture	Third-party	2×107/m2 × 2 doses	CMV: CR
	Hyper IgM Syndrome	CMV	CMV	Culture	Third-party	2×107/m2 × 2 doses	CMV: CR
	SCID	Adenoviremia	Adv	Culture	Third-party	2×107/m2	Adv- CR
	SCAEBV	EBV	EBV	Culture	Third-party	2×107/m2	PD, Died (Day+14)

Table abbreviations: CR: complete response; PR: partial response; NR: no response; PD: progressive disease; NA: Not available; NE: Not evaluated; DC: dendritic cells; LCL: lymphoblastoid cell line.

In the CMV setting, Bao et al. administered CMV-specific VSTs generated using peptide expansion to treat 7 patients who had refractory CMV post HSCT. Three of these patients had underlying PIDD diagnoses and of these patients, 2 of the had a complete virologic response.³⁸ The non-responding patient had recurrence of viremia in the setting of corticosteroid use for relapsing hemophagocytic lymphohistiocytosis (HLH), and eventually died of fungal pneumonia.

Several phase 1 trials have evaluated immunomagnetic selection to generate VSTs to prevent and treat serious viral infections in transplant patients with both malignant and non-malignant diseases, including primary immunodeficiency disorders. Feuchtinger et al. generated adenovirus specific VSTs which were administered to 9 patients (one with PIDD), for adenoviremia refractory to multiple drugs post HSCT.(30) Five of six evaluable patients had complete responses, including resolution of adenoviral infection in the patient with PIDD. A follow-up study by the same group treated 30 patients for adenoviremia, of whom 7 patients had immunodeficiency. They achieved clearance of adenovirus in 14 and partial responses in 7 patients, though further breakdown by diagnosis was not provided.(27) The difference in adenovirus related mortality in VST responders versus non-responders was significant- 9.5% versus 100%, respectively. In a recent French multi-institutional study led by Necker Hospital for Sick Children, Creidy et al. treated 15 patients with CMV, adenovirus, or both viruses with VSTs targeting a specific virus.(31) Of these patients, 5 had PIDDs. Three of the PIDD patients were treated with CMV-specific VSTs, 1 with adenovirus (Adv)-specific VSTs, and one patient received both CMV and Adv-specific VSTs. Three of these patients had virologic responses (2 to CMV and 1 to Adv) and were alive at follow-up. Two of these patients had stabilization of viral retinitis, while one had vision loss (Table 1). The two non-responding patients died of pulmonary disease (one following treatment of CMV, and one following treatment of Adv).

Multiviral VSTs in PIDD

The majority of VST studies over the past decade have administered VST products specific for multiple viruses using different ex vivo expansion manufacturing approaches.^15,16,21 Leen et al from Baylor College of Medicine treated a patient with SCID with trivirus specific T-cells generated using EBV-transformed B cells transduced with an Ad5f35CMVpp65 vector to stimulate and expand EBV/CMV/Adv specific T cells. The patient with SCID received these VSTs for Adv infection and achieved a complete response.(16) In a follow-up study by the same group, donor-derived VSTs with specificity to EBV and Adv were given to patients following HSCT. In this study, one patient with SCID was treated prophylactically. Expansion and persistence of EBV-specific T-cells was observed in the peripheral blood of this patient post infusion, corresponding with a lack of viral reactivations or infections.(15) Subsequently, this group produced VSTs targeting EBV, CMV, and Adv antigens utilizing a rapid expansion methodology where antigen presenting cells nucleofected with plasmids encoding viral antigens (Hexon/Penton for Adv, IE1/pp65 for CMV and LMP2/BZLF1 for EBV) were used to stimulate and expand multivirus specific T cells.¹³ These tri-specific VSTs were used to treat 10 patients, including 1 patient with HLH who had dual infection with CMV and adenovirus. This patient achieved a complete response against both viruses.⁶ In a recent retrospective study pulling together the combined VST experience at Baylor College of Medicine, Children’s National, Newcastle University, and the University of Aberdeen, Naik et al. described 13 previously unpublished patients with PIDD who received donor-derived VSTs after HSCT.(39) Eight of these patients were treated prophylactically and had no viral infections or reactivations post VSTs. Five patients were treated for CMV or EBV with complete viral clearance achieved in four patients post VSTs. The group at Baylor College of Medicine (Papadopoulou et al.) subsequently used APC pulsed with overlapping peptide pools specific for 12 different viral antigens to generate VSTs with activity against 5 viruses: EBV, CMV, adenovirus, BK virus, and HHV6.¹² Eleven patients were treated on this study, including 4 with PIDD who received VSTs as treatment for up to 3 concurrent viral infections. At 12 weeks post-infusion, 3 of the 4 PIDD patients achieved a complete virologic response. The fourth patient achieved a complete response to EBV and HHV6, but no response to BK virus which was related to the fact that the VST product infused did not have specificity for BK virus (Table 1).

Third-party derived VSTs in PIDD

Though donor-derived VSTs have been very successful in prior trials, use of third-party VSTs from specialty T-cell banks eliminates the time period required for production and the need for an available donor, and has the potential to vastly widen availability of this therapy. There is limited data on third-party VSTs specifically for patients with PIDD. However, several studies to date have evaluated use of third-party VSTs in patients with a variety of diagnoses that have demonstrated efficacy in the setting of treatment-refractory viral disease (Table 1). The group at the University of Aberdeen (Vickers et al.) described 3 patients with PIDD who were treated for EBV-LPD with third party EBV-specific T cells, one of whom achieved a complete response.(33) The group at MSKCC (Doubrovina et al.) similarly described one patient with X- linked lymphoproliferative syndrome who received third party EBV-specific VSTs for EBV-LPD post HSCT, achieving a complete remission.(24) In the multicenter retrospective analysis by Naik et al, 4 previously unpublished patients with PIDD received third-party VST for post-HSCT viral infections. One of these patients cleared both adenovirus and CMV. Two patients died of progressive EBV-LPD, and another had a partial response to CMV, but later died of fungal pneumonia. The group from Westmead Institute and the University of Sydney (Withers et al.) similarly studied the use of partially HLA-matched “off the shelf” VSTs in 30 patients.(38) Three of these patients had PIDD; 2 were treated for adenovirus, and one patient was treated for EBV. All three patients achieved a complete response and were alive 12 months post-infusion (Table 1).

Though the majority of VSTs have been administered to patients post HSCT, viral infections are also a serious complication prior to HSCT in patients with SCID and other profound forms of PIDD. Several case reports have described the use of VST in this setting. Wynn et al. from the University of Edinburgh used third party EBV-specific VSTs for the treatment of a patient (who was later shown to have CTPS1 deficiency(39)) with primary CNS, EBV-associated lymphoma who had a poor initial response to chemotherapy.(40) The patient received 2 VST infusions resulting in a significant clinical improvement and undetectable levels of EBV viremia, allowing the patient to proceed to HSCT. The patient did have EBV reactivation post HSCT, but again achieved a complete response with rituximab and two additional third-party VST infusions. Uhlin et al. at Karolinska University described the use of maternally-derived VSTs targeting a dominant HLA-A02:01-restricted epitope on CMV-pp65 (NLVPVATM) to treat an infant with SCID and drug-refractory CMV infection.(41) A partial virologic response was seen with 10-fold decrease in viral load, but the infant died after subsequent umbilical cord blood transplantation. Miller et al. described the use of third party adeno-specific VSTs manufactured at Children’s National for a patient with RAG1 SCID and disseminated adenoviremia treated under eIND at Phoenix Children’s Hospital.(42) Prior to transplant, the patient received two doses of third party VSTs that were 3/10 HLA matched with the patient, followed shortly thereafter by HSCT from a matched unrelated donor. Nineteen days after the second VST infusion (11 days post HSCT), the patient had undetectable levels of adenovirus.

Safety of VSTs in PIDD

Many previous phase I studies and case reports have shown VSTs to be safe. In the current literature, infusion reactions are rarely described, and were always mild when reported.(43) It is likely these mild reactions are likely secondary to preservatives used during cryopreservation (e.g. DMSO) or the concomitant use of benadryl, rather than the T cells per se. While most phase I studies do describe a number of adverse events, they are usually not attributable to the infusion, and are likely a reflection of the high morbidity population that is usually in need of infusions. The main concern with T cell infusions is potential alloreactivity of the product however, multiple reports suggest that GVHD and CRS rates are very low.(10, 44)

Graft versus host disease (GvHD)

The most commonly discussed potential risk following T-cell infusion is the development of graft versus host disease post VST infusion. As previously described(44), current methods of VST manufacture decrease the overall risk of GvHD by infusing mostly, if not all, effector cells, therefore reducing alloreactive cells in the final product. In a multicenter study, Leen et al. reported that of the 50 patients treated with third-party VST, 8 (16%) developed GvHD post T cell infusion.(37) GvHD was low grade in 7/8 patients (stage I in 6, stage II in 1) with one grade III liver GVHD developing in a patient with disseminated adenovirus disease. Most (75%) of the patients who reported GVHD post VSTs had a history of GvHD prior to T cell infusion. Most published studies report a similarly low incidence of graft versus host disease in the donor-specific and third party VST settings.(21, 23, 24) When GVHD has developed post T cell infusion, it is typically low grade and tends to easily resolve with therapy.(39) Interestingly, the incidence of GvHD is not appreciably different between studies when donor-derived versus third-party derived VST products are administered.(45) It is important to note, however, that current data is predominantly available from phase I and retrospective studies which lack control groups. Therefore, it remains yet to be proven if this rate is in fact different from what would be expected in the general post HSCT population.

Cytokine Release Syndrome (CRS)

CRS is a form of systemic inflammatory response syndrome, caused by a large, rapid release of cytokines into the blood from immune cells.(46) When severe, it can present with high fevers, hypotension, and multi-organ dysfunction. Given the high rates of cytokine release syndrome seen with chimeric-antigen receptor (CAR) therapy, it stands to reason this could be a potential risk of VSTs.(47) To date, cases of CRS have been very rare after VST infusion, though it has been described, particularly in cases with high viral burden.(48) There are 2 reports of worsening respiratory and hepatic status post VSTs, both in the setting of severe adenoviremia.(31, 49) These were considered potentially attributable to the T cell infusion, though these are also well described virus-associated complications in the immunocompromised host. There are also two reports of respiratory failure after EBV-specific T cell infusions, both in the context of an inflammatory reaction to bulky EBV disease in the upper chest.(14) It is therefore advisable to monitor for CRS post infusion, particularly in cases with high viral loads or bulky disease.

Graft Rejection/Allosensitization

Potential allosensitization against HLA proteins is a big concern in the bone marrow transplant population. Graft rejection is a particular concern with use of third party VSTs that may be partially mismatched with the stem cell donor, but this has not been seen in the many trials reporting the use of third-party VSTs in this setting. Allosensitization is also a theoretical concern when administering third party VSTs for patients with PIDD prior to HSCT, but considering the underlying immunodeficiency present in patients with forms of PIDD that warrant transplantation, this theoretical risk is thought to be low. Miller et al. described secondary graft loss in a patient with RAG1 SCID who underwent unconditioned marrow infusion from a matched unrelated donor after receiving third party VST therapy for adenovirus. Retransplantation utilizing the same donor after preconditioning chemotherapy was successful, suggesting that engraftment failure was not related to VST infusion and most likely related to the lack of conditioning chemotherapy before the first transplant(42)

Study Limitations to date

Though VST therapy has proved efficacious for treatment and prevention of viral infections in immunocompromised patients, there are still limitations moving forward. Few studies of VST therapy conducted thus far have included control groups,(17, 24) so it has been difficult to assess the true effect on survival rates. As studies move forward in the multicenter setting using multi-viral specific T-cells, the question of antigenic dominance when targeting a large number of viral antigens has been raised. Depending on the donor serostatus and HLA type, VST products may be heavily skewed to highly immunodominant viral antigens for one or maybe two viruses. For example, CMV seropositive donors expressing B07:01 invariably have a robust response against CMV restricted through this HLA allele, which may account for a skewing of the expanded T-cell product to this single viral epitope.(45, 50) Viral specificity is also dependent upon pre-existing immunity in the donors. This, along with turn-around time to manufacture the VSTs, has led to the use of third party VSTs. However, as previously noted, these products are only partially HLA matched and are usually rejected within 3 months of administration.(37, 41) Given the lack of randomized, controlled trials, it is also difficult to fully assess the safety risks, including GVHD and CRS. The use of VSTs is promising, and addressing these study limitations will be important as the use of VSTs broadens applicability and becomes a treatment mainstay for refractory infections in the immunocompromised host.

FUTURE OF VST THERAPY FOR PIDD

The key future milestones for VST therapy in the PIDD patient population include widening availability and improving the breadth of pathogens that can be targeted. Studies to date have predominantly targeted DNA viruses such as CMV, EBV, and adenovirus. However, many other viruses are a potential threat to patients with profound PIDD. Respiratory viruses in particular are a danger, including RSV, human metapneumovirus, and human parainfluenza virus.(9, 51) T-cell epitopes for many of these viruses have been previously described,(52–54) and a current phase I trial is underway to test VST therapy targeting human parainfluenza virus-3. Among other potential targets, Human papilloma virus (HPV) is problematic in many PIDD patients with T-cell deficiencies, and is described even after HSCT in patients with X-linked and JAK3 deficient SCID.(55, 56) T-cell expansion targeting HPV proteins E6 and E7 has been successful from heathy donors, but has not been tested clinically to date.(57)

Many questions remain with respect to the differing manufacturing approaches for VST generation. Both primate and human studies have highlighted the importance of progenitor T- cell populations including central memory and stem cell memory subsets to maintain antigen- specific responses in vivo.(58–60) These populations are detectable in VST products at low percentages, but could potentially be enhanced using cell selection techniques or by modulating expansion conditions in vitro. For example, IL-21 and inhibitors of glycogen synthase 3b have both been described to maintain precursor T-cell populations.(61, 62) Hence, favoring these precursors could potentially improve persistence of VSTs in vivo.

Third party VST therapy holds great promise for extending availability of VSTs as an “off the shelf” therapeutic, but algorithms for best VST product selection in the HLA mismatched setting are not yet broadly established. Confirmation of the HLA restrictions which mediate antiviral activity however appears to be essential for antiviral activity.(36, 45) The stability of viral antigens in circulating viral isolates is also a critical factor for VST therapy, as an escape mutation has been described in the setting of adoptive immunotherapy targeting EBV.(63) Finally, as the number of targeted pathogens increases, it has been theorized that there may be an upper limit to the number of antigens that can be successfully targeted in a single T cell product due to antigenic competition. In studies published to date, this has seemingly not been a major issue even targeting up to 5 viruses in a monoculture. However, it stands to reason that the number of cells targeting an individual antigen will be diluted by adding more antigens to a culture.

The cost of adoptive immunotherapy is another topic that has drawn much scrutiny, particularly with regard to chimeric antigen receptor (CAR) T cells.(64) At our institution, we have recently determined that the cost of manufacturing and testing a single VST product to be $8500. This is comparable in cost to a course of many intravenous antiviral medications. Upon eventual approval, it is likely that the commercial cost will be higher, but as VSTs are not individualized products and do not require vector manufacturing or transduction, it is unlikely that third party VST banks will ever approach the cost of CAR T-cell therapy.

Administration of VSTs has been highly successful as a prophylactic and treatment strategy for high risk PIDD patients after HSCT. As we better define the effector and progenitor populations that are essential for adaptive immunity, modular transplantation is likely to represent the future of definitive therapy for PIDD, and VSTs will likely be an important tool in this field. An improved understanding of ideal viral targets and potential toxicities, particularly with the use of VSTs prior to HSCT, will be essential to further improve and expand adoptive T-cell therapy for patients with PIDD.