In our ASK VRS series, every month we will reply to a question asked through our website or social media. This month, Ali sent us the following question:

Can I search for polyomavirus specific IgG and IgM antibodies in cancer patients’ serum or, because they are immunocompromised, they cannot show antibody titer? And is the condition the same for patients with hematological malignancy?

 Defense from viral diseases depends mainly on the immune system. When immunity is compromised, common viruses that rarely cause problems among the healthy population can trigger severe illness and complicated outcomes.

Over recent decades, due to increased use of intensive antitumor therapy, more organ transplants, and the HIV epidemic, the number of immunosuppressed and immunocompromised individuals has been growing. Such acquired immune deficiencies are driving an increase in the incidence of serious infections.

As we might expect, those with reduced immunity are at increased susceptibility to common community-acquired pathogens. But this is not their only vulnerability; these individuals are also vulnerable to reactivation of endogenous latent organisms. Most people (even the healthiest among us) harbour multiple latent viruses; these lie dormant within the body and can be reactivated later in life. This reactivation can be triggered in multiple ways (e.g., by immunosuppression and inflammation), but is more likely in those with a suboptimal immune system.

Also, new diseases associated with immune deficiencies have appeared. For example, post-transplant lymphoproliferative disease (PTLD) is a lymphoma mostly induced by the Epstein-Barr virus (EBV) in patients taking immune system suppressing drugs following a transplant or those with immune deficiencies. In people with a robust immune response, EBV is relatively harmless, causing infectious mononucleosis (pain, sore throat, fever, jaundice, AKA ‘the kissing disease’).

Another example is progressive multifocal leukoencephalopathy (PML), which was a clinical rarity in the past, but has now become an important cause of neurological complications. PML is caused by human polyomavirus JC in HIV patients. From the above, we can appreciate the importance of the host immune system in the control of latent infections and opportunistic diseases. Some viruses that are significantly associated with disease in immunocompromised hosts include EBV, Cytomegalovirus (CMV), Herpes simplex viruses (HSV1, HSV2), Varicella zoster virus (VZV), Hepatitis B and C virus (HBV, HBC), and Human Polyomaviruses (HPyVs). These infections can be dangerous in immunocompromised patients because, in the absence of infection-fighting white blood cells, the infection can rapidly progress from fever to sepsis, and ultimately death. In such cases, prompt and accurate identification of the viral pathogen is critical, and an early diagnosis is essential to enable aggressive treatments with specific antiviral therapy.

Human polyomaviruses

Human polyomaviruses (HPyVs) are small, non-enveloped, double-stranded DNA viruses of the family Polyomaviridae. HPyVs are prevalent in the general population but rarely cause clinically apparent pathology, and disease only manifests in immune-compromised individuals. Among HPyVs, the most relevant are JCPyV, BKPyV, MCPyV, and Trichodysplasia spinulosa PyV (TSPyV), which share common characteristics (e.g., genome size and organization) but are unique in their tissue tropism and associated pathology. HPyVs can mediate cell transformation and tumorigenesis in immunodeficient hosts, but can also be the cause of various other conditions, such as hemorrhagic cystitis, nephropathy, and progressive multifocal leukoencephalopathy (PML, a neurological disease). Cancer patients (including those with hematological malignancies) receiving chemotherapy are immunosuppressed (an unintended side-effect of the treatment). As a result, these patients often fail to generate an adequate antibody response to infection. Therefore, serologic testing for antibodies against a specific virus is generally not useful in the acute management of immunocompromised patients. Because of the frequency of infection and the risk of severe disease in the immunologically compromised host, rapid laboratory detection of polyomavirus has become an important issue in diagnostic virology.

So, how do you diagnose an infection by specific pathogens in patients that do not mount an immune response against the invader?

The answer: you have to search for the virus in itself. In this approach, you can either isolate the virus using cell culture or use multiple diagnostic methods to detect viral antigens, such as viral nucleic acids or specific viral proteins through PCR-based molecular tests and antigen assays. In past times, viral isolation through cell culture was the technique of choice. Nowadays, this practice is mostly used for viruses that grow rapidly in the laboratory (e.g., HSV) making culture-based testing practical. While viral isolation has an increasingly less important role as modern immunologic and molecular tests provide more rapid results, in some instances, viral isolation is still necessary because it is the only technique capable of providing a viable isolate that can be used for further characterization and that allows the detection of multiple viruses, including those unsuspected at the time when the culture was requested. This approach has been most widely used for HSV, HIV, CMV, adenovirus, RSV, influenza and parainfluenza viruses, rhinovirus, and enteroviruses. However, for the detection of all viruses, more rapid antigen and nucleic acid detection tests are gradually replacing viral culture methods.

Antigen detection

Methods of antigen detection include fluorescent antibody staining and enzyme immunoassays (e.g., ELISA). Antigen-detection techniques are rapid and do not require viable virus to be present in the specimen, allowing more flexibility in the handling and transport of samples. Antigen detection methods are particularly useful for viruses that grow slowly, making their recovery from culture difficult. The most important targets in antigen detection have been RSV, influenza and parainfluenza viruses in respiratory specimens; HSV and VZV in cutaneous specimens; and CMV and hepatitis B virus in blood specimens. Viruses such as the enteroviruses and the rhinoviruses that have extensive antigenic heterogeneity and lack cross-reacting antigens are not suitable for antigen-detection techniques.

Nucleic acid detection

By PCR, it is possible to diagnose viral infection through sensitive detection of specific viral nucleic acids. Any virus can potentially be detected in this way! Traditional methods for detecting and identifying HPyVs include virus isolation by cell culturing and electron microscopy. More recently, PCR analysis has enabled sensitive and specific detection of these viruses in a different range of clinical samples. The PyVs genome displays a great deal of sequence variation between the various subtypes making the versatile PCR the ideal technique for specific PyV detection.