**5. Nondestructive vs. monomorphic PTLD**

Destructive PTLD, which has typically not correlated with other specific risk factors, has recently been shown to be associated with older recipient age and prolonged receipt of calcineurin inhibitors. Furthermore, recent data has contributed to and, in some instances, shed light on previous debate concerning the role of viruses other than EBV and the level of HLA mismatch as risk factors for PTLD [15]. Gene association studies focusing on key cytokines and their receptors have identified several polymorphisms that may prove useful to identify patients at risk, with distinction for early nondestructive and late occurring monomorphic PTLD.

**8. Role of immunosuppression (IST)**

complications and increased mortality.

**9. Other rare subtypes of PTLD**

T-cell lymphomas with or without EBV infection.

**8.1. ATG and PTLD**

immunosuppression.

**9.1. Burkitt PTLD**

nism is lost, therefore promoting the development of PTLD.

The degree and duration of immunosuppression play a major role in the development of PTLD. Cytotoxic T cells provide a defense mechanism against EBV-infected B cells in immunocompetent individuals. However, when T-cell function is impaired, this defense mecha-

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In vivo T-cell depletion (TCD) with antithymocyte globulin (ATG) or alemtuzumab (AL) is commonly used in HSCT. TCD facilitates engraftment and reduces the incidence and severity of graft-versus-host disease (GvHD). As reduced intensity conditioning (RIC) and matched unrelated donor transplants (MUD) are now being performed more frequently, ATG and AL have become integral components of preparative regimens. Although ATG and AL provide safer T-cell depletion, delayed T-cell reconstitution following TCD accounts for infectious

EBV PTLD is predominantly derived from donor B cells before reconstitution of the EBVspecific cytotoxic T lymphocyte (CTL) response. It can, however, occur later in the most severely immunocompromised patients with additional risk factors such as donor and recipient mismatch, graft manipulation with T-cell depletion, as well as the degree and duration of

In a study by Buyck et al., an overall incidence of 6.3% for EBV PTLD was reported in 89 patients with severe aplastic anemia. A marked increase in the incidence (13.3%) was noted in

Although the majority of monomorphic PTLDs fall into the category of diffuse large B-cell lymphoma, other types of lymphoma may also occur. These include plasma cell myeloma, classical Hodgkin lymphoma, or HL-like PTLD, EBV+ MALT lymphoma as well as various

A variety of lymphomas can develop as PTLD, although some types appear infrequently and remain poorly understood. PTLD-Burkitt lymphomas behave aggressively and require intensive chemotherapeutic intervention. These display the typical histological features of Burkitt lymphoma but are markedly positive for EBV. While BL-PTLD is a rare entity, it is a discrete form of PTLD with a high EBV expression and should be treated as a high-grade lymphoma. BL-PTLD historically represents a small, but significant, proportion of PTLD cases. BL-PTLD represents 15% of PTLD patients for pediatric heart, lung, and heart-lung transplants from

patients exposed to ATG, with 5 of 43 patients developing EBV PTLD [19].
