**1. Introduction—Measles Virus for Cancer Therapy**

Measles virus (MeV) is a negative-strand RNA virus belonging to the family Paramyxoviridae, genus *Morbillivirus*. Its genome has a length of approximately 16 kb and encodes six structural and two non-structural proteins (Figure 1a,b). The viral glycoproteins hemagglutinin and fusion mediate receptor binding and fusion at the plasma membrane, respectively. While wild type MeV uses CD150/SLAM on lymphoid cells and epithelial nectin-4 as receptors, vaccine strains of MeV infect cells primarily via CD46 [1]. This is due to mutations in the receptor attachment protein hemagglutinin H in vaccine strain MeV, resulting in high affinity of H for CD46 [2–6]. MeV infection results in syncytia formation as typical cytopathic effect (Figure 1c).

**Citation:** Engeland, C.E.; Ungerechts, G. Measles Virus as an Oncolytic Immunotherapy. *Cancers* **2021**, *13*, 544. https://doi.org/ 10.3390/cancers13030544

Academic Editor: David Wong Received: 31 December 2020 Accepted: 26 January 2021 Published: 1 February 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

[15,17].

preclinical studies.

**Figure 1.** Measles as an oncolytic virus. (**a**) Schematic of the measles virus particle. The viral RNA genome is encapsulated by the nucleocapsid (N) protein and is associated with the viral polymerase (L, large protein) and its cofactor phosphoprotein (P), forming the ribonucleoprotein complex (RNP). The matrix (M) protein connects the RNP and the viral envelope. The surface glycoproteins hemagglutinin (H) and fusion (F) mediate receptor binding and cell fusion, respectively. (**b**) Schematic of the measles virus genome with open reading frames encoding the six structural proteins flanked by the 3′ leader (ld) and 5′ trailer (tr). (**c**) Syncytia formation as the typical cytopathic effect associated with measles virus infection. Human colorectal cancer (KM12, top) and pancreatic adenocarcinoma (T3M4, bottom) cells were transfected with plasmids encoding the MeV glycoproteins H and F as well as enhanced green fluorescent protein as reporter. Control cells were subjected to mock transfection. Phase contrast and fluorescence images were acquired with an Axiovert 200 microscope (Zeiss) at 36 h (KM12) and 12 h post-transfection (T3M4). Scale bar: 200 µM. (**d**) Lymphoma remission after measles infection. Left panel: The patient presented with orbital Burkitt's lymphoma. Middle panel: The patient was infected with measles; the typical skin rash is visible. Right panel: Without specific anti-lymphoma treatment, the orbital mass resolved. Reproduced from Lancet 10 July 1971; 2 (7715): 105–106, with permission. **Figure 1.** Measles as an oncolytic virus. (**a**) Schematic of the measles virus particle. The viral RNA genome is encapsulated by the nucleocapsid (N) protein and is associated with the viral polymerase (L, large protein) and its cofactor phosphoprotein (P), forming the ribonucleoprotein complex (RNP). The matrix (M) protein connects the RNP and the viral envelope. The surface glycoproteins hemagglutinin (H) and fusion (F) mediate receptor binding and cell fusion, respectively. (**b**) Schematic of the measles virus genome with open reading frames encoding the six structural proteins flanked by the 30 leader (ld) and 50 trailer (tr). (**c**) Syncytia formation as the typical cytopathic effect associated with measles virus infection. Human colorectal cancer (KM12, top) and pancreatic adenocarcinoma (T3M4, bottom) cells were transfected with plasmids encoding the MeV glycoproteins H and F as well as enhanced green fluorescent protein as reporter. Control cells were subjected to mock transfection. Phase contrast and fluorescence images were acquired with an Axiovert 200 microscope (Zeiss) at 36 h (KM12) and 12 h post-transfection (T3M4). Scale bar: 200 µm. (**d**) Lymphoma remission after measles infection. Left panel: The patient presented with orbital Burkitt's lymphoma. Middle panel: The patient was infected with measles; the typical skin rash is visible. Right panel: Without specific anti-lymphoma treatment, the orbital mass resolved. Reproduced from Lancet 10 July 1971; 2 (7715): 105–106, with permission.

> apy. Advantages of MeV include the excellent safety profile of the oncolytic vaccine strains and lack of genotoxicity, its immunogenicity, and especially the plethora of engineering possibilities offered by the MeV reverse genetics system. Specific challenges related to MeV include pre-existing antiviral immunity, the choice of preclinical models and manufacturing. These assets and drawbacks are discussed in more detail within this review article. **2. Measles Virus Oncotropism**  Measles vaccine strain oncotropism correlates with CD46 overexpression on malignantly transformed cells [23]. Although viral entry occurs in benign cells and at low CD46 Originally, the idea to treat cancer patients with MeV arose after case reports which linked measles infection to tumor remission [7]. One highly cited example relates to a boy suffering from Burkitt's lymphoma [8] (Figure 1d). These experiments of nature inspired the idea of using MeV in cancer treatment. However, measles is a severe infectious disease [9]. Thus, employing a pathogenic strain of MeV in cancer therapy is out of question. Live attenuated MeV strains for vaccination were licensed in the 1960s and have a proven safety record [10]. Several years later, testing of Edmonston B measles vaccine strain derivatives for cancer treatment began. In many early studies, hematological malignancies were chosen as target entities [11–14]. This was supported by the natural lymphotropism of MeV. However, other malignancies including ovarian cancer [15] and glioblastoma [16] were soon found to also be sensitive to MeV oncolysis, while normal cells are spared [15,17].

Thus, MeV is one of several oncolytic platforms currently developed for cancer ther-

safety record [10]. Several years later, testing of Edmonston B measles vaccine strain derivatives for cancer treatment began. In many early studies, hematological malignancies were chosen as target entities [11–14]. This was supported by the natural lymphotropism of MeV. However, other malignancies including ovarian cancer [15] and glioblastoma [16] were soon found to also be sensitive to MeV oncolysis, while normal cells are spared

Meanwhile, preclinical efficacy of oncolytic MeV has been demonstrated against a broad range of cancer entities (reviewed in [18]). In addition to Edmonston B derivatives, also the vaccine strains Moraten-Schwarz [19], Edmonston-Zagreb and AIK-C [20], rMV-Hu191 [21], as well as Leningrad-16 [22] have been shown to exert oncolytic effects in

receptor density, a certain threshold of expression is required for syncytia formation and cell death [24]. In myeloma, CD46 upregulation has been associated with abnormal p53 [25]. The epithelial receptor for MeV, nectin-4 [26,27], is also a tumor marker which may render carcinomas of pancreatic [28], colorectal [29], and mammary [30] origin susceptible to MeV oncolysis. Post-transcriptional regulation of nectin-4 levels by miR-31 and miR-Meanwhile, preclinical efficacy of oncolytic MeV has been demonstrated against a broad range of cancer entities (reviewed in [18]). In addition to Edmonston B derivatives, also the vaccine strains Moraten-Schwarz [19], Edmonston-Zagreb and AIK-C [20], rMV-Hu191 [21], as well as Leningrad-16 [22] have been shown to exert oncolytic effects in preclinical studies.

Thus, MeV is one of several oncolytic platforms currently developed for cancer therapy. Advantages of MeV include the excellent safety profile of the oncolytic vaccine strains and lack of genotoxicity, its immunogenicity, and especially the plethora of engineering possibilities offered by the MeV reverse genetics system. Specific challenges related to MeV include pre-existing antiviral immunity, the choice of preclinical models and manufacturing. These assets and drawbacks are discussed in more detail within this review article.
