T-Cell and NK-Cell Large Granular Lymphocyte Proliferations

History and Classification

The first detailed studies of what is now recognized as T-cell large granular lymphocytic leukemia (T-LGL) were published in the 1970s and early 1980s. These manuscripts described a disorder associated with neutropenia or anemia that was associated with a proportionate and absolute increase in circulating granular lymphocytes that were reactive with antisera to T-cells, cell that normally constituted only 10% to 20% of the peripheral blood lymphocytes. These studies also documented that the cytoplasmic granules of these lymphocytes were identical in ultrastructure to those of their normal counterpart (parallel microtubular arrays); formed sheep erythrocyte rosettes consistent with T-cell origin (now recognized as CD2 positivity); and expressed Fc receptors. Based on these attributes, the moniker large granular lymphocytic leukemia was coined. It should be noted, however, that this disorder was ascribed a number of different names in the early literature, including CD8-positive T-cell chronic lymphocytic leukemia and T-gamma lymphoproliferative disorder .

Our understanding of leukemias of large granular lymphocytes greatly advanced in the late 1980s and 1990s with the recognition of cytotoxic T cells and natural killer cells (NK cells) as discrete lymphocyte subsets and the advent of multicolor flow-cytometric immunophenotyping, which allowed these cell types to be distinguished and characterized in clinical specimens. During this period, T-cell large granular lymphocytic leukemia (T-LGL) became the widely accepted nomenclature, with the fundamental defining attributes being an increase in granular lymphocytes with a CD8-positive T-cell phenotype, aberrant expression of the NK-cell lineage– associated antigens CD16 and CD57, and T-cell clonality as documented by the presence of clonal T-cell receptor gene rearrangements. Cases fulfilling these criteria often were associated with neutropenia and typically had an indolent clinical course. Also recognized during this time period were cases in which the increased granular lymphocytes had a CD3-negative, CD16-positive, CD56-positive NK-cell immunophenotype. Rendering a diagnosis of large granular lymphocytic leukemia in such cases was more problematic, however, as there were few methods to establish NK-cell immunophenotypic aberrancy, and NK-cell clonality could not be readily assessed because these cells lack T-cell receptor gene rearrangement. For these reasons, during this period the diagnosis of large granular lymphocytic leukemia of NK-cell lineage (NK-LGL) required a greater degree of clinical morbidity to confidently distinguish such cases from a potential reactive NK-cell lymphocytosis. Therefore, NK-LGL was considered to be more aggressive than its T-cell counterpart in these earlier reports.

More recently, further advances have improved our ability to identify chronic lymphoproliferative disorders of cytotoxic T cells and NK cells and can also provide potential insights into pathogenesis, including the identification of a novel family of receptors for MHC-I and related proteins that are expressed by NK cells and a subset of cytotoxic T cells (referred to as natural cytotoxicity receptors, NCRs ). Through the application of these tools, T-LGL is now recognized as a disorder of memory cytotoxic T cells variably associated with cytopenias, which typically has an indolent clinical course. An NK-cell–derived counterpart with similar clinical and laboratory features, including surrogate markers of clonality, has also been elucidated. This is now referred to as chronic lymphoproliferative disorder of NK cells (CLPD-NK) to distinguish it from the earlier NK-LGL descriptions, which likely included more aggressive NK-cell malignancies and are described elsewhere in this text.

Definition

T-cell large granular lymphocytic leukemia is defined as a clonal or oligoclonal increase in peripheral blood cytotoxic T cells with granular lymphocyte morphology. Cytopenias are usually associated, yet they are not universally present and are not a defining attribute, per se. Although increased circulating granular lymphocytes is the quintessential feature of T-LGL, the use of an absolute LGL count as a diagnostic criterion has changed over time. In early disease definitions, an absolute LGL count of 2 × 10 ⁹ /L was used. However, subsequently bona fide T-LGL not reaching this threshold were identified and, over time, it has come to be recognized that up to one third of cases may have a count lower than 1 × 10 ⁹ /L. For this reason, an absolute LGL count is no longer included as a disease-defining feature, although granular lymphocytes compose greater than 50% of the circulating lymphoid cells in most cases.

The T cells of T-LGL are typically CD8-positive, alpha-beta type, although cases that are either CD4 positive or of gamma-delta lineage may uncommonly be encountered. In virtually all cases, phenotypic abnormalities are present, with co-expression of NK-associated antigens CD16 and/or CD57 considered pathognomic but not disease-specific. In the vast majority of T-LGLs, T-cell clonality can be detected through either molecular analysis of T-cell receptor gene rearrangements or V-beta flow cytometry. Given the oligoclonal nature of the disease, however, clonality may not be demonstrated in all cases. In such cases, documented persistence of the process by repeat studies after a period of 6 months to 1 year before rendering an unequivocal diagnosis is prudent.

CLPD-NK is also defined by an increase in peripheral blood granular lymphocytes (also associated with cytopenias in some instances); however, in this disorder the lymphocytes are of NK-cell lineage as documented by flow cytometry. As in T-LGL, in CLPD-NK large granular lymphocytes compose the majority of the peripheral blood lymphocytes, and, although the elevation in the absolute count is usually mild, it tends to be slightly higher than that seen in T-LGL. NK-cell immunophenotypic aberrancy is variably attributed as a feature of CLPD-NK. This variability likely reflects the limited NK-cell phenotyping, which is routinely used in many clinical laboratories as aberrancy can be demonstrated in all cases when extensive NK-cell immunophenotyping including antibodies to NCRs is performed. Because NK cells lack T-cell receptor gene rearrangements demonstrating clonality, evaluation of these genes is not part of the routine diagnostic evaluation or disease definition. The lack of a readily assessed marker of clonality in CLPD-NK places a greater emphasis on documenting persistence of the process for 6 months to 1 year. As in T-LGL, cytopenias are frequently associated but not part of the disease definition.

Bone marrow immunohistochemistry revealing intrasinusoidal cytotoxic marrow infiltrates is detected in 75% or more of T-LGL and CLPD-NK cases. Likewise, recent studies demonstrated that STAT3 mutation is present in approximately 50% of both T-LGLs and CLPD-NKs. Therefore, these features are coming to be accepted as defining characteristics of these conditions, although neither is specific for T-LGL or CLPD-NK.

Etiology and Epidemiology

Antigenic stimulation is considered a primary etiologic event in both T-LGL and CLPD-NK, although no singular causative agent or predisposing factors have been identified for either condition. Serologic studies have demonstrated that in 30% or more of T-LGLs and CLPD-NKs, antibodies to HTLV-1 envelope proteins p21 and p24 are present in the absence of detectable HTLV-1 or HTLV-2 viral DNA. These findings suggest a role for infection by an HTLV-related virus in the development of some cases. In T-LGL, analysis of T-cell receptor beta chain variable region gene usage has revealed similar clonotypes among cases and within the oligoclonal expansions of individual cases, and also disproportionate use of the TCR Vb 13.1, which is physiologically expanded in response to CMV infection. These data further implicate viral infection as a potential etiologic agent in T-LGL. Other stimulants of cytotoxic T cells may also play a role in T-LGL development as demonstrated by the association of this disorder with other hematolymphoid neoplasms.

CMV infection was also shown to cause oligoclonal expansion of killer-cell immunoglobulin-like receptor (KIR)-expressing NK cells, and the activating form of the KIR appears important in physiologic responses to viral infection. In CLPD-NK, a disproportionate number have KIR haplotypes that are rich in activating isoforms, and there is frequent expression of these activating KIRs and epigenetic inactivation of the inhibitory KIR genes. These data suggest that viral infection may also be an important etiologic factor in the development of CLPD-NK. CLPD-NK is also associated with other stimulants of cellular immunity such as other neoplasms, but this has not been documented to the same degree as in T-LGL.

T-LGL and CLPD-NK are epidemiologically similar. Both appear rare, with T-LGL representing less than 5% of all mature lymphoid leukemias and CLPD-NK about one third as common as T-LGL. These figures may not be indicative of the true prevalence of these diseases, however, as these diagnoses require both a high level of clinical suspicion and comprehensive laboratory evaluation, and therefore both may be underrecognized.

T-LGL and CLPD-NK are diseases of adulthood, both with a median age of 50 to 60 years. Only sporadic cases were described in adolescents and young adults. Neither shows a predilection for gender or are EBV-associated. Although some studies suggest that T-LGL may be slightly more common in Asian populations, neither shows a strong geographic or ethnic predisposition.

Clinical Features

T-LGL is strongly associated with both autoimmune phenomenon and autoimmune disorders. Although the estimates vary between studies, it can be reliably stated that over 30% of T-LGLs have abnormal serologic studies associated with immune activation such as polyclonal hypergammaglobulinemia, the presence of detectable rheumatoid factor, or a positive ANA. Approximately 20% of T-LGL patients have clinically diagnosed rheumatoid arthritis. A number of other autoimmune diseases are also associated with T-LGL with lesser frequency, including systemic lupus erythematosus, chronic inflammatory bowel disease, and Sjögren's syndrome.

In addition to being associated with autoimmune disorders, T-LGL also has the somewhat unusual feature of being associated with other clonal hematologic disorders, which are present in 10% and 20% of cases. B-cell lineage lymphoproliferative disorders are most commonly described in this context, with the majority of these having immunophenotypic features of chronic lymphocytic leukemia and representing this disorder or, more often, the subclinical condition monoclonal B-cell lymphocytosis. Among the other hematologic diseases that have been identified in association with T-LGL are plasma cell proliferative disorders (including monoclonal gammopathy of uncertain significance), Hodgkin's lymphoma, chronic myelomonocytic leukemia, and hairy cell leukemia.

Lastly, case reports of a T-LGL are being diagnosed in the setting of allogeneic renal and bone marrow transplantation. The precise nature of such cases is unclear, however, as reactive CD8-positive T-cell expansions with limited clonal diversity and phenotypically similar to T-LGL have been described following allogeneic transplantation, in HIV infection, and with certain pharmacotherapies including dasatinib treatment for chronic myelogenous leukemia. Therefore, it is difficult to determine whether these cases represent bona fide lymphoproliferative disorders or distinct reactive processes of limited clonal diversity.

CLPD-NK is also associated with autoimmune disease and other hematologic malignancies. These associations do not appear to be as strong as those seen in T-LGL, however, with autoimmune disease present in less than 25% of cases and only sporadic instances of association with other clonal hematolymphoid conditions including plasma cell proliferative disorders and Hodgkin's lymphoma.

In general, neither T-LGL nor CLPD-NK is associated with B symptoms or significant clinical morbidity. Typically, patients present with signs or symptoms related to the disease-associated cytopenias (neutropenic infection or anemia-associated fatigue and dyspnea on exertion), and oftentimes these diagnoses are made during evaluation of asymptomatic lymphocytosis. The primary clinically evident sign in these disorders is organomegaly, particularly splenomegaly. The reported frequency of clinically appreciable splenomegaly in T-LGL and CLPD-NK varies, and in some studies is reported to be as high as 50% for T-LGL and greater than 90% for CLPD-NK. These higher frequencies are seen in older studies, however, and likely reflect both preferential identification of cases with higher disease burden and the inclusion of disease entities other than T-LGL and CLPD-NK. In evaluating more recent literature, splenomegaly is present in approximately 20% to 30% of T-LGLs, and it is slightly less prevalent in CLPD-NK. Hepatomegaly has also been described in both conditions, again more frequently in older studies. Overall, hepatomegaly is present in approximately 10% of T-LGL and CLPD-NK cases. In these LGL disorders, the organomegaly is presumably due to organ infiltration, although when hepatomegaly is present it is not usually associated with hepatic dysfunction. Mild lymphadenopathy may be seen in isolated cases. Prominent lymphadenopathy or involvement of extramedullary tissue sites is not typical, however, and, if present, should lead to the consideration of other possible diagnoses.