Introduction

According to the Global Observatory of Donation and Transplantation, 115,000 solid-organ transplants were performed in 2012 worldwide, of which 68% were kidney transplants . Improved surgical techniques and more effective immunosuppressive drugs have led to a substantial rise in post-transplant graft survival – with 1-year survival exceeding 90% in kidney recipients, and >80% for heart, liver or lung recipients. However, several long-term complications such as cardiovascular diseases, nephrotoxicity, infections and malignancies still limit the graft and patient’s survival . In fact, as the risk of acute rejection diminishes, the frequency of malignancies increases in parallel with a longer duration of immunosuppression, which not only impairs the immune function but also has a direct oncogenic activity .

It has been estimated that the overall incidence of malignancy is now about 20% after 10 years of iatrogenic immunosuppression . Accordingly, the longer life expectancy, the exposure to immunosuppressive drugs and the progressive ageing of organ recipients will likely bring oncologic diseases to represent the main cause of mortality in recipients of solid organs in the coming decades .

Several studies have shown that transplanted patients, when compared to the general population, experience a two-to fivefold increase in de novo malignancies (DNMs), but they are not at a generalized cancer risk. The most common neoplastic types registered in the general population (i.e., carcinomas of the lung, breast, prostate and colon) are, actually, relatively uncommon in transplant patients. Conversely, the most common DNMs are relatively rare tumours in the general population and are often associated with infections sustained by oncogenic viruses, such as non-Hodgkin lymphoma (NHL) or Kaposi’s sarcoma (KS) with up to hundredfold augmented risks . From a clinical viewpoint, most DNMs are diagnosed at advanced stages, and are characterized by aggressive behaviour . Women of reproductive age are growing in number among the recipients of solid-organ transplants. Although the monitoring of pregnancy outcomes after transplant has shown that successful pregnancies are possible in the transplant population , specific data on cancer outcomes are lacking.

Recipients of solid-organ transplants (in addition to human immunodeficiency virus (HIV)-infected patients) constitute an original setting for studying cancers under immunological control, including the interaction of known risk factors with acquired immune deficiency. This article aims to give an update on the epidemiological aspects of DNMs after organ transplantation.

Cancer pathogenesis in recipients of solid-organ transplant and role of immunosuppression

Cancer development is a multifactorial process, and a competent immune system recognizes and attempts to eliminate primary tumours through the action of cytotoxic T lymphocytes, macrophages and natural killer cells, which can recognize tumour cells as non-self (the so-called immunosurveillance), delay tumour progression and prevent angiogenesis, vascular invasion and metastasis . The immune system also exerts a control on viral infections with oncogenic capacity, but in transplant recipients, immunosuppressive drugs disrupt the immune function and have direct effects on the site of tumour formation by promoting cell transformation and escape from immune recognition . However, this effect is largely and rapidly reversible when the reduction of immunosuppression occurs, as shown in kidney recipients returning to dialysis .

The drug dose intensity is a key factor in the anti-rejection strategy . For instance, the risk of developing DNMs among thoracic organ (heart/lung) recipients is two-to fourfold higher than among liver- and kidney-transplanted patients, which is probably related to the required higher dose of immunosuppression used to prevent rejection. Likewise, a higher dose of calcineurin inhibitors, that is, tacrolimus or cyclosporine in the first 30 days after liver transplant, resulted in a higher rate of recurrence of hepatocellular carcinoma (HCC) . Induction with polyclonal anti-lymphocyte globulins, or some monoclonal antibodies like anti-CD3 (muromonoab-CD3 or OKT3), but not anti-interleukin-2 (IL-2) receptor antibodies, has been shown to increase the risk of post-transplant lymphoproliferative disorders (PTLDs) – NHL in particular .

Concerning the direct pro-oncogenic activity of anti-rejection drugs, in vitro studies have demonstrated that this effect may be promoted by neo-angiogenesis induced by over-expression of the transforming growth factor beta (TGF-β) . Other authors have demonstrated that an increased expression of vascular endothelial growth factor or calcineurin inhibitors is able to induce the production of IL-6, which activates B lymphocytes, their growth and presumably their immortalization . Patients treated with these drugs showed, in fact, a higher incidence of PTLDs. Purine metabolism inhibitor azathioprine (AZA) shows a potential carcinogenic effect, once partially converted into thioguanine, as it decreases DNA reparation efficiency and increases microsatellite instability within cells .

Conversely, another nucleotide analog mycophenolate mofetil (MMF), as well as its precursor mycophenolic acid, appears to have a protective role against some DNMs, based on the anti-proliferative and anti-apoptotic effect against large B-cell NHL . Other studies have reported, however, an increased incidence of KS in patients treated with MMF , though regression of post-transplant KS after conversion from cyclosporine to MMF has been documented .

Mammalian targets of rapamycin inhibitors (mTORi) show anti-tumour capacity and represent a major breakthrough, as they are related to a decreased risk of cancer development. Clinical studies have reported a significant reduction of post-transplant malignancies in patients treated with mTORi . In addition, a regression of PTLD, KS and of non-melanoma skin cancer (NMSC) has also been observed in patients who after diagnosis of malignancy switched their treatment protocol to rapamycin , which showed significant anti-rejection activity, as well as blocking activity, against tumour cell growth and invasiveness, both in vivo and in vitro . Furthermore, rapamycin can revert the transformed phenotype of immortalized cells through different mechanisms – including cell proliferation kinase stimulation inhibition , induction of E-cadherin expression by diminishing metastatization of tumour cells or inhibiting the production of angiogenic factors . Another mTORi, everolimus, closely related to rapamycin, has an anti-proliferative effect on B lymphocytes infected with Epstein–Barr virus (EBV), both in vitro and in animal models . Rapamycin and its analogs inhibit B-cell proliferation induced by EBV infection through the inhibition of IL-10 secretion, which play an important role in the development of this type of tumour .

Cancer pathogenesis in recipients of solid-organ transplant and role of immunosuppression

Cancer development is a multifactorial process, and a competent immune system recognizes and attempts to eliminate primary tumours through the action of cytotoxic T lymphocytes, macrophages and natural killer cells, which can recognize tumour cells as non-self (the so-called immunosurveillance), delay tumour progression and prevent angiogenesis, vascular invasion and metastasis . The immune system also exerts a control on viral infections with oncogenic capacity, but in transplant recipients, immunosuppressive drugs disrupt the immune function and have direct effects on the site of tumour formation by promoting cell transformation and escape from immune recognition . However, this effect is largely and rapidly reversible when the reduction of immunosuppression occurs, as shown in kidney recipients returning to dialysis .

The drug dose intensity is a key factor in the anti-rejection strategy . For instance, the risk of developing DNMs among thoracic organ (heart/lung) recipients is two-to fourfold higher than among liver- and kidney-transplanted patients, which is probably related to the required higher dose of immunosuppression used to prevent rejection. Likewise, a higher dose of calcineurin inhibitors, that is, tacrolimus or cyclosporine in the first 30 days after liver transplant, resulted in a higher rate of recurrence of hepatocellular carcinoma (HCC) . Induction with polyclonal anti-lymphocyte globulins, or some monoclonal antibodies like anti-CD3 (muromonoab-CD3 or OKT3), but not anti-interleukin-2 (IL-2) receptor antibodies, has been shown to increase the risk of post-transplant lymphoproliferative disorders (PTLDs) – NHL in particular .

Concerning the direct pro-oncogenic activity of anti-rejection drugs, in vitro studies have demonstrated that this effect may be promoted by neo-angiogenesis induced by over-expression of the transforming growth factor beta (TGF-β) . Other authors have demonstrated that an increased expression of vascular endothelial growth factor or calcineurin inhibitors is able to induce the production of IL-6, which activates B lymphocytes, their growth and presumably their immortalization . Patients treated with these drugs showed, in fact, a higher incidence of PTLDs. Purine metabolism inhibitor azathioprine (AZA) shows a potential carcinogenic effect, once partially converted into thioguanine, as it decreases DNA reparation efficiency and increases microsatellite instability within cells .

Conversely, another nucleotide analog mycophenolate mofetil (MMF), as well as its precursor mycophenolic acid, appears to have a protective role against some DNMs, based on the anti-proliferative and anti-apoptotic effect against large B-cell NHL . Other studies have reported, however, an increased incidence of KS in patients treated with MMF , though regression of post-transplant KS after conversion from cyclosporine to MMF has been documented .

Mammalian targets of rapamycin inhibitors (mTORi) show anti-tumour capacity and represent a major breakthrough, as they are related to a decreased risk of cancer development. Clinical studies have reported a significant reduction of post-transplant malignancies in patients treated with mTORi . In addition, a regression of PTLD, KS and of non-melanoma skin cancer (NMSC) has also been observed in patients who after diagnosis of malignancy switched their treatment protocol to rapamycin , which showed significant anti-rejection activity, as well as blocking activity, against tumour cell growth and invasiveness, both in vivo and in vitro . Furthermore, rapamycin can revert the transformed phenotype of immortalized cells through different mechanisms – including cell proliferation kinase stimulation inhibition , induction of E-cadherin expression by diminishing metastatization of tumour cells or inhibiting the production of angiogenic factors . Another mTORi, everolimus, closely related to rapamycin, has an anti-proliferative effect on B lymphocytes infected with Epstein–Barr virus (EBV), both in vitro and in animal models . Rapamycin and its analogs inhibit B-cell proliferation induced by EBV infection through the inhibition of IL-10 secretion, which play an important role in the development of this type of tumour .

Pregnancy outcomes in solid-organ transplant recipients

Nowadays, pregnancy represents a realistic opportunity after solid-organ transplantation, and it is estimated that one in every 50 women of childbearing age will become pregnant after undergoing organ transplantation. To the best of our knowledge, there are no studies in the literature reporting on cancer risk in pregnancy associated with the anti-rejection immunosuppressive therapy. According to the National Transplantation Pregnancy Registry, the main topics of the debate on pregnancy after organ transplantation are as follows: the course of pregnancy, the function and survival of the transplanted organ and the effects of the immunosuppressive agents on the developing foetus, including the long-term outcome of the infant . Studies performed on kidney, liver or heart transplant recipients showed a percentage around 70% of infants born alive, and the most frequent complications reported include hypertension and maternal infections, preterm delivery and low birth weight of the newborn infant. However, these pregnancies are regarded as being at high risk and require a multidisciplinary approach. To date, the outcomes of the children followed up have been encouraging, and specific remote effects have not been identified, but continued surveillance is still vital. Of special concern are the new immunosuppressive drugs, specifically for MMF, where data reported to the National Transplantation Pregnancy Registry and through post-marketing surveillance have shown an increased incidence of non-viable outcomes and a specific pattern and increased incidence of malformation in the newborn .

Infection and post-transplant malignancies

Approximately one out of five incident cancers worldwide can be attributed to infectious agents . The impairment of immunosurveillance is supposed to ease the expansion of virus-infected cells, through either a new viral infection or a reactivation of a latent one . The implicit ability of several viruses to immortalize infected cells by disrupting the cell-cycle control, in a setting of induced lowered immunosurveillance, could lead to tumourigenesis .

Tumours linked to oncogenic viruses are potentially more immunogenic through the presentation of viral peptides than tumours arising from chemical or environmental carcinogenesis, which may not as readily be recognized as non-self. In the immunosuppression scenario, the immune system loses the ability to recognize and kill tumour cells, but when the host immune system is restored the growth of virus-associated tumours may be reversed. In immunosuppressed individuals, some tumours can often regress as a result of a withdrawn or reduced immunosuppressive therapy (in transplant recipients) or of highly active antiretroviral therapy .

Several viruses predispose transplant recipients to specific malignancies ( Table 1 ), and the majority of tumours developing preferentially in immunocompromised patients are often associated with infections with oncogenic viruses such as EBV, KS herpesvirus (KSHV), human papillomavirus (HPV) or hepatitis viruses B (HBV) and C (HCV) , which the immunosuppressed host is unable to control and restrain.

Infection with EBV is a typical example of a cellular transformation-inducing virus. The International Agency for Research on Cancer classified EBV as a Group 1 carcinogen and concluded that there is sufficient evidence for a causative role of EBV in nasopharyngeal cancer, endemic Burkitt’s lymphoma, immunosuppression-related NHL, extranodal T-cell lymphoma and a subset of Hodgkin lymphomas, while there is limited evidence for the role of EBV in gastric carcinoma and lympho-epithelioma-like carcinoma .

Primary infection with EBV promotes a latent phase in which the EBV genome is maintained in an episomal form in the infected cell and its progeny, through the action of EBV nuclear antigens 1 and 2 and transmembrane molecule LMP-1 (latent membrane protein 1). The latter leads to the production of important transcription factors and of anti-apoptotic proteins ensuring the immortalization of infected cells .

In transplanted patients, EBV-driven tumour growth could result from the reactivation of latent persistent EBV infection (especially in the first months after the transplant, when immunosuppression is higher), or as the effect of an uncontrolled EBV primary infection in an EBV-negative recipient, typically found in paediatric patients where, actually, PTLDs represent by far the major group of DNMs .

KSHV has been evaluated as a Group 1 carcinogen, with adequate evidence supporting a necessary role for this infection in the pathogenesis of KS, and of a few rare lymphoproliferative disorders, such as primary effusion lymphoma and multicentre Castleman’s disease . KSHV has been shown to induce the formation, in primary endothelial cell cultures, of spindle cells, which are considered as the neoplastic component of KS, and to reduce their dependence on growth factors . Like all other herpesviruses, KSHV establishes a latent gene expression programme in the majority of infected endothelial spindle cells in KS, avoiding immune response by minimizing the production of antigenic proteins. However, a small proportion of infected cells in these tumours still reflect a productive infection.

HPV is a non-enveloped double-stranded DNA virus that infects basal epithelial cells, causing benign and malignant lesions of the skin and mucosa. More than 100 different HPV genotypes (or ‘types’) have been identified so far; among them, a dozen are classified as ‘high risk’ (e.g., types 16 or 18) for their causal association with cervical cancer and other mucosal anogenital cancers (penile, anal and vulvar cancers), while, by contrast, the low-risk types are associated with benign lesions such as genital warts . About 10% of women infected with HPV – especially if infected with high-risk types – will develop a persistent infection, a prerequisite for the development of cervical cancer. Persistent infection is accompanied by integration of the viral genome into the human DNA and constitutive production, in the selected clones, of early viral genes such as E6 and E7, able to interact with tumour-suppressor proteins, thus abrogating normal DNA damage responses and contributing to the accumulation of genetic alterations that promote tumour growth and inhibit apoptosis, ultimately leading to the progression to cancer . Immunosuppression seems to be important in facilitating HPV replication and/or persistence by disrupting the local immunosurveillance, but it apparently has a lower effect in promoting progression to cancer. HPV infection is also associated with other malignancies, such as Bowen’s disease, tonsillar cancer and some types of oropharyngeal squamous cell carcinomas, which are found at an increased risk in kidney- and liver-transplanted patients .

Several studies highlighted an association of HPV with cutaneous squamous cell carcinoma (SCC), but its real role as a causative agent for SCC is still a controversial issue . HPV is highly prevalent in immunosuppressed patients, and it is more likely found in SCC lesions than in normal skin, even if this does not imply causality. HPV could be involved in initiation of the oncogenesis process, rather than tumour promotion or maintenance. HPV may act as a co-carcinogen with other factors to amplify the risk of developing SCC. Studies have shown that HPV DNA is more prevalent in sun-exposed versus sun-protected skin, also suggesting a link between the two factors; moreover, it has been postulated that HPV could perturb cellular DNA repair or apoptosis mechanisms, leading the cells to become more susceptible to ultraviolet (UV)-induced damage. UV light may have a transient immunosuppressive effect on skin allowing HPV to evade the immune system. Its higher prevalence in SCC lesions could therefore be merely a marker and not a cause of immunosuppression .

Donor transmission of malignancies

Transmission of malignancies from the donor with undiagnosed cancers at the time of donation is a rare event with a negligible considered incidence (<1% of all transplant from deceased donor in USA) . Available information on donor-derived malignancy transmission is limited to anecdotal reports, with 0.5–3% of donors having a history of cancer, and its transmission has been demonstrated in 0.02–6% of these cases . The rate of transmission is high for donors with renal cell carcinoma (RCC), melanoma and choriocarcinoma; intermediate for lung, colon and breast cancers; and low for donors with skin cancers, in situ or low-grade tumours. The risk of transmission also depends on which organ is transplanted with a low to high gradient for liver, kidney, heart and lung, respectively. A comprehensive categorization of the risk of transmission (from minimal to high) for all tumours has been recently proposed by the Disease Transmission Advisory Committee of the Organ Procurement and Transplantation Network (OPTN)/United Network for Organ Sharing (UNOS) .

Several international organizations have defined a list of recommendations for malignancy screening, which include a complete history of neoplasia, physical examination of skin malignancies, blood testing for the detection of tumour markers (recommending the storage of samples for future analysis), imaging testing, pathological examination of procured organs and routine autopsy; moreover, unexplained intracranial haemorrhage or menstrual disorders in women should be investigated to rule out the presence of underlying oncological diseases .

A malignancy in a transplant recipient does not generally result from the direct transmission of the malignancy itself, but rather from the transmission of infectious viral diseases, which in the immune-compromised state of the transplant recipient could result in acute infections, transformation and progression to tumour .

DNMs could be diagnosed from donor cells (donor-derived) – PTLD of donor cell origin, as an example – or late-onset neoplasms that arise from allograft donor cells. In the case of late tumours (such as 10 years post transplant), it is reasonable that no mass lesion existed at the time of transplant, and for this reason only tumours originating from donor cells arising within 2 years post transplant have to be considered donor-transmitted tumours .

Other risk factors

The improvement of surgical techniques and clinical treatment protocols allowed so far a considerable increase in life expectancy of transplant recipients, resulting in a protracted exposure to risk factors or to potentially carcinogenic agents. In addition, the risk of developing cancer increases with ageing. The accumulation of DNA mutations through the years, the decreased efficiency of repair systems and the impaired ability of the immune system to deal with the insults coming from the environment are important risk factors for further development of tumours .

In the case of kidney transplant recipients, some genetic conditions, for example, end-stage polycystic kidney disease which requires kidney transplantation, is associated with an augmented risk of skin cancer and renal or genitourinary malignancies . In addition, the prolonged period spent on dialysis in the waiting list for transplant enables the easy acquisition of infections with potentially oncogenic viruses .

Genetic factors and the history of tumours are known to predispose to the development of malignancies. According to one study, patients who had an invasive carcinoma before transplantation had a much higher risk of developing a second invasive carcinoma de novo after transplantation .

Commonly known risk factors such as cigarette smoking, alcohol and analgesic abuse or sun exposure can as well facilitate post-transplant malignancies . Studies in patients undergoing liver transplantation have shown that subjects with previous alcohol abuse, or alcoholic cirrhosis as transplantation indication, have a higher risk of cancer (especially of the gastrointestinal tract) after liver transplantation with a worse prognosis . The abuse of alcohol and cigarette smoking appears to increase the risk of cancer, particularly lung and oropharyngeal cancers in liver transplant patients . It must be remembered that acute alcohol intoxication promotes tumourigenesis by decreasing the activity of natural killer cells in tumour surveillance .

Excessive sun exposure is an additional risk factor particularly for immunocompromised patients, and it is considered as the main cause of skin tumours . The risk of developing skin cancer is associated with the degree of sun exposure in the geographical area of interest and genetic characteristics of the individual (light-colour eyes, fair skin or hair) . The chronic use of immunosuppressive drugs (especially steroids) causes, among others, a strong sensitivity to light increasing the carcinogenic effects of sunlight exposure .