Diseases of White Blood Cells, Lymph Nodes, Spleen, and Thymus

Pathogenesis

Neutropenia can be caused by (1) inadequate or ineffective granulopoiesis or (2) increased destruction or sequestration of neutrophils in the periphery. Inadequate or ineffective granulopoiesis is observed in the setting of:

• Suppression of HSCs, as occurs in aplastic anemia ( Chapter 14 ) and a variety of infiltrative marrow disorders (e.g., tumors, granulomatous disease); in these conditions, granulocytopenia is accompanied by anemia and thrombocytopenia.

• Suppression of committed granulocytic precursors by exposure to certain drugs (discussed later).

• Disease states associated with ineffective hematopoiesis, such as megaloblastic anemia ( Chapter 14 ) and myelodysplastic syndrome, in which defective precursors die in the marrow.

• Rare congenital conditions (e.g., Kostmann syndrome), in which inherited defects in specific genes impair granulocytic differentiation.

Accelerated destruction or sequestration of neutrophils occurs with:

• Immunologically mediated injury to neutrophils, which can be idiopathic, associated with a well-defined immunologic disorder (e.g., systemic lupus erythematosus), or caused by exposure to drugs.

• Splenomegaly, in which splenic enlargement leads to sequestration and destruction of neutrophils in the spleen and modest neutropenia, sometimes associated with anemia and often with thrombocytopenia.

• Increased peripheral utilization, which can occur in overwhelming bacterial, fungal, or rickettsial infections.

The most common cause of agranulocytosis is drug toxicity . Certain drugs, such as alkylating agents and antimetabolites used in cancer treatment, produce agranulocytosis in a predictable, dose-related fashion. Because such drugs cause a generalized suppression of hematopoiesis, production of red cells and platelets is also affected. Agranulocytosis may also occur as an idiosyncratic reaction to a large variety of agents including certain antibiotics, anticonvulsants, antiinflammatory drugs, antipsychotic drugs, and diuretics. The neutropenia induced by antipsychotic agents such as chlorpromazine and related phenothiazines results from a toxic effect on granulocytic precursors in the bone marrow. In contrast, agranulocytosis following administration of other drugs, such as sulfonamides, probably stems from antibody-mediated destruction of neutrophils through mechanisms similar to those involved in drug-induced immunohemolytic anemias ( Chapter 14 ).

In some patients with acquired idiopathic neutropenia, autoantibodies directed against neutrophil-specific antigens are detected. Severe neutropenia may also occur in association with monoclonal proliferations of large granular lymphocytes (so-called LGL leukemia ). The mechanism of this neutropenia is not clear; suppression of granulocytic progenitors by products of the neoplastic cell (usually a CD8+ cytotoxic T cell) is considered most likely.

Morphology

The alterations in the bone marrow vary with cause. With excessive destruction of neutrophils in the periphery, the marrow is usually hypercellular due to a compensatory increase in granulocytic precursors. Hypercellularity is also the rule with neutropenias caused by ineffective granulopoiesis, as occurs in megaloblastic anemia and myelodysplastic syndrome. Agranulocytosis caused by agents that suppress or destroy granulocyte precursors is understandably associated with marrow hypocellularity.

Infections are a common consequence of agranulocytosis. Ulcerating necrotizing lesions of the gingiva, floor of the mouth, buccal mucosa, pharynx, or elsewhere in the oral cavity (agranulocytic angina) are quite characteristic. These are typically deep, undermined, and covered by gray to green-black necrotic membranes from which numerous bacteria or fungi can be isolated. Less frequently, similar ulcerative lesions occur in the skin, vagina, anus, or gastrointestinal tract. Severe life-threatening invasive bacterial or fungal infections may occur in the lungs, urinary tract, and kidneys. The neutropenic patient is at particularly high risk for deep fungal infections caused by Candida and Aspergillus. Sites of infection often show a massive growth of organisms with little leukocytic response. In the most dramatic instances, bacteria grow in colonies (botryomycosis) resembling those seen on agar plates.

Clinical Features

The symptoms and signs of neutropenia are related to infection and include malaise, chills, and fever, often followed by marked weakness and fatigability. With agranulocytosis, infections are often overwhelming and may cause death within hours to days.

Serious infections are most likely when the neutrophil count falls below 500/mm ³ . Because infections are often fulminant, broad-spectrum antibiotics must be given expeditiously whenever signs or symptoms appear. In some instances, such as following myelosuppressive chemotherapy, neutropenia is treated with G-CSF, a growth factor that stimulates the production of granulocytes from marrow precursors.

Pathogenesis

The peripheral blood leukocyte count is influenced by several factors, including:

• The size of the myeloid and lymphoid precursor and storage cell pools in the bone marrow, thymus, circulation, and peripheral tissues.

• The rate of release of cells from the storage pools into the circulation.

• The proportion of cells that are adherent to blood vessel walls at any time (the marginal pool).

• The rate of extravasation of cells from the blood into tissues.

As discussed in Chapter 3 , leukocyte homeostasis is maintained by cytokines, growth factors, and adhesion molecules through their effects on the proliferation, differentiation, and extravasation of leukocytes and their progenitors. Table 13.2 summarizes the major mechanisms of neutrophilic leukocytosis and its causes, the most important of which is infection. In acute infection there is a rapid increase in the egress of mature granulocytes from the bone marrow pool, an alteration that may be mediated through the effects of tumor necrosis factor (TNF) and interleukin-1 (IL-1). If the infection or an inflammatory process is prolonged, IL-1, TNF, and other inflammatory mediators stimulate macrophages, bone marrow stromal cells, and T cells to produce increased amounts of hematopoietic growth factors. These factors enhance the proliferation and differentiation of committed granulocytic progenitors and, over several days, cause a sustained increase in neutrophil production.

Some growth factors preferentially stimulate the production of a single type of leukocyte. For example, IL-5 mainly stimulates eosinophil production, while G-CSF induces neutrophilia. Such factors are differentially produced in response to various pathogenic stimuli, and, as a result, the five principal types of leukocytosis (neutrophilia, eosinophilia, basophilia, monocytosis, and lymphocytosis) tend to be observed in different clinical settings ( Table 13.3 ).

In sepsis or severe inflammatory disorders (e.g., Kawasaki disease), leukocytosis is often accompanied by morphologic changes in neutrophils, such as toxic granulations, Döhle bodies, and cytoplasmic vacuoles ( Fig. 13.2 ). Toxic granules, which are coarser and darker than normal neutrophilic granules, represent abnormal azurophilic (primary) granules. Döhle bodies are patches of dilated endoplasmic reticulum that appear as sky-blue cytoplasmic “puddles.”

In most instances it is not difficult to distinguish reactive and neoplastic leukocytoses, but uncertainties may arise in two settings. Acute viral infections, particularly in children, can cause the appearance of large numbers of activated lymphocytes that resemble neoplastic lymphoid cells. At other times, particularly in severe infections, many immature granulocytes appear in the blood, mimicking a myeloid leukemia (leukemoid reaction) . Special laboratory studies (discussed later) are helpful in distinguishing reactive and neoplastic leukocytoses.

Pathogenesis

The common feature of all forms of HLH is systemic activation of macrophages and CD8+ cytotoxic T cells. The activated macrophages phagocytose blood cell progenitors in the marrow and formed elements in the peripheral tissues, while the “stew” of mediators released from macrophages and lymphocytes suppress hematopoiesis and produce symptoms of systemic inflammation. These effects lead to cytopenias and a shock-like picture, sometimes referred to as “cytokine storm” or the systemic inflammatory response syndrome ( Chapter 4 ).

Familial forms of HLH are associated with several different mutations, all of which impact the ability of cytotoxic T cells (CTLs) and natural killer (NK) cells to properly form or deploy cytotoxic granules. How these defects lead to HLH is not known. One common trigger for HLH is Epstein-Barr virus infection, suggesting that in some instances HLH stems from a defect in the ability of CD8+ CTLs to kill infected cells. As a result of the persistent infection, the CTLs continue to make cytokines, leading to excessive macrophage activation. HLH is also a common complication of peripheral T-cell lymphoma (discussed later), a tumor of mature T cells that is marked by immune dysregulation. Regardless of the trigger, HLH is uniformly associated with extremely high levels of inflammatory mediators such as interferon-γ, TNF-α, IL-6, and IL-12.

Clinical Features

Most patients present with an acute febrile illness associated with splenomegaly and hepatomegaly. Hemophagocytosis is usually seen on bone marrow examination, but is neither sufficient nor required to make the diagnosis. Laboratory studies typically reveal anemia, thrombocytopenia, and very high levels of plasma ferritin and soluble IL-2 receptor, both indicative of severe inflammation, as well as elevated liver function tests and triglyceride levels, both related to hepatitis. Coagulation studies may show evidence of disseminated intravascular coagulation. If untreated, this picture can progress rapidly to multiorgan failure, shock, and death.

Treatment involves the use of immunosuppressive drugs, “mild” chemotherapy, and administration of an antibody that neutralizes the activity of interferon-γ. Patients with germline mutations that cause HLH or who have persistent/resistant disease are candidates for HSC transplantation. Without treatment, the prognosis is grim, particularly in those with familial forms of the disease, who typically survive for less than 2 months. With prompt treatment, with or without subsequent HSC transplantation, roughly half of patients survive, though many do so with significant sequelae, such as renal damage in adults and growth retardation and intellectual disability in children.

Pathogenesis

Many of the chromosomal aberrations seen in ALL dysregulate the expression and function of transcription factors required for normal B- and T-cell development. Most T-ALLs have mutations in NOTCH1, a gene that is essential for T-cell development, while a high fraction of B-ALLs have mutations affecting genes such as PAX5, TCF3, ETV6, and RUNX1, all of which are required for the proper differentiation of early hematopoietic precursors. By disturbing the expression and function of “master” regulatory factors, these mutations promote maturation arrest and increased self-renewal, a stem cell–like phenotype. Similar themes are relevant in the genesis of AML (discussed later).

In keeping with the multistep origin of cancer ( Chapter 7 ), mutations in transcription factor genes are not sufficient to produce ALL. The identity of other driver mutations is incomplete, but aberrations that promote cell growth, such as mutations that increase tyrosine kinase activity and RAS signaling, are commonly present. Emerging data from deep sequencing of ALL genomes suggest that fewer than 10 mutations are sufficient to produce full-blown ALL; hence compared to solid tumors, ALL is genetically simple.

Approximately 90% of ALLs have numerical or structural chromosomal changes. Most common is hyperploidy (>50 chromosomes), but hypoploidy and a variety of balanced chromosomal translocations also are seen. Changes in chromosome numbers are of uncertain pathogenic significance but are important because they frequently correlate with immunophenotype and sometimes prognosis. For example, hyperdiploidy and hypodiploidy are seen only in B-ALL and are associated with better and worse prognoses, respectively. In addition, B-ALL and T-ALL are associated with completely different sets of translocations; thus, while morphologically identical, they are genetically quite distinct.

Morphology

In leukemic presentations, the marrow is hypercellular and packed with lymphoblasts, which replace normal marrow elements. Mediastinal thymic masses occur in 50% to 70% of T-ALLs, which are also more likely than B-ALL to be associated with lymphadenopathy and splenomegaly. In both B-ALL and T-ALL, the tumor cells have scant basophilic cytoplasm and nuclei somewhat larger than those of small lymphocytes ( Fig. 13.6A ). The nuclear chromatin is delicate and finely stippled, and nucleoli are usually small and often demarcated by a rim of condensed chromatin. The nuclear membrane is often deeply subdivided, imparting a convoluted appearance. In keeping with the aggressive clinical behavior, the mitotic rate is high. As with other rapidly growing lymphoid tumors, interspersed macrophages ingesting apoptotic tumor cells may impart a “starry sky” appearance (shown in Fig. 13.15 ).

Because of their different responses to chemotherapy, ALL must be distinguished from AML, a neoplasm of immature myeloid cells that can cause identical signs and symptoms. Compared with myeloblasts, lymphoblasts have more condensed chromatin, less conspicuous nucleoli, and smaller amounts of cytoplasm that usually lacks granules. However, these morphologic distinctions are not absolute, and definitive diagnosis relies on stains performed with antibodies specific for B- and T-cell antigens ( Fig. 13.6B and C ). Histochemical stains are also helpful, in that (in contrast to myeloblasts) lymphoblasts are myeloperoxidase-negative and often contain periodic acid–Schiff–positive cytoplasmic material.

Clinical Features

Although ALLs and AMLs are genetically and immunophenotypically distinct, they are clinically very similar. In both, the accumulation of neoplastic “blasts” in the bone marrow suppresses normal hematopoiesis by physical crowding, competition for growth factors, and other poorly understood mechanisms. The common features and those more characteristic of ALL are the following:

• Abrupt stormy onset within days to a few weeks of the first symptoms.

• Symptoms related to depression of marrow function, including fatigue due to anemia; fever, reflecting infections secondary to neutropenia; and bleeding due to thrombocytopenia.

• Mass effects caused by neoplastic infiltration (which are more common in ALL), including bone pain resulting from marrow expansion and infiltration of the subperiosteum; generalized lymphadenopathy, splenomegaly, and hepatomegaly; testicular enlargement; and, in T-ALL, complications related to compression of large vessels and airways in the mediastinum.

• Central nervous system manifestations such as headache, vomiting, and nerve palsies resulting from meningeal spread, all of which are also more common in ALL.

Treatment of pediatric ALL is one of the great success stories of oncology. With aggressive chemotherapy about 95% of children with ALL obtain a complete remission, and 75% to 85% are cured. Despite these achievements, however, ALL remains a leading cause of cancer deaths in children, and only 35% to 40% of adults are cured. Several factors are associated with a worse prognosis: (1) age younger than 2 years, largely because of the strong association of infantile ALL with translocations involving the MLL gene; (2) presentation in adolescence or adulthood; and (3) peripheral blood blast counts greater than 100,000, which probably reflects a high tumor burden. Favorable prognostic markers include (1) age between 2 and 10 years; (2) a low white cell count; (3) hyperdiploidy; (4) trisomy of chromosomes 4, 7, and 10; and (5) the presence of a t(12;21). Notably, the molecular detection of residual disease after therapy is predictive of a worse outcome in both B-ALL and T-ALL and is being used to guide clinical trials.

Although most chromosomal aberrations in ALL alter the function of transcription factors, the t(9;22) instead creates a fusion gene that encodes a constitutively active BCR-ABL tyrosine kinase (described in more detail under Chronic Myeloid Leukemia ). In B-ALL, the BCR-ABL protein is usually 190 kDa in size and has stronger tyrosine kinase activity than the form of BCR-ABL that is found in chronic myeloid leukemia, in which a BCR-ABL protein of 210 kDa in size is usually seen. Treatment of t(9;22)-positive ALLs with BCR-ABL kinase inhibitors in combination with conventional chemotherapy is highly effective and has greatly improved the outcome for this molecular subtype of B-ALL in children and adults. Of interest, cryptic rearrangements involving genes encoding tyrosine kinases other than ABL also have been described in B-ALL, particularly in adults, and these too may be therapeutic targets. The outlook for adults with ALL lacking “targetable” molecular lesions remains more guarded, in part because of differences in the molecular pathogenesis of adult and childhood ALL, and also because older adults cannot tolerate the intensive chemotherapy regimens that are curative in children. Dramatic responses in B-ALL have been achieved with chimeric antigen receptor T cells directed against the B-cell antigen CD19 ( Chapter 7 ), but at high cost and with associated toxicities that have sometimes proved fatal.

Key Concepts

Acute Lymphoblastic Leukemia/Lymphoblastic Lymphoma

• Most common type of cancer in children; may be derived from either precursor B or T cells.

• Highly aggressive tumors manifest with signs and symptoms of bone marrow failure or as rapidly growing masses.

• Tumor cells contain genetic lesions that block differentiation, leading to the accumulation of immature, nonfunctional blasts.

• A subset of tumors contains activating mutations in tyrosine kinases (e.g., BCR-ABL) that are important targets of therapy.

Pathogenesis

Unlike most other lymphoid malignancies, chromosomal translocations are rare in CLL/SLL. The most common genetic anomalies are deletions of 13q14.3, 11q, and 17p and trisomy 12q. Molecular characterization of the region deleted on chromosome 13 has implicated two microRNAs, miR-15a and miR-16-1, tumor suppressor genes. Loss of these mIRs is believed to result in overexpression of the anti-apoptotic protein BCL2, which is uniformly observed in CLL/SLL. DNA sequencing has revealed that the Ig genes of some CLL/SLL are somatically hypermutated, whereas others are not, suggesting that the cell of origin may be either a post–germinal center memory B cell or a naive B cell. For unclear reasons, tumors with unmutated Ig segments (those putatively of naive B-cell origin) pursue a more aggressive course. Deep sequencing of CLL genomes has also revealed gain-of-function mutations involving the NOTCH1 receptor in 10% to 18% of tumors, as well as frequent mutations in genes that regulate RNA splicing.

The growth of CLL/SLL cells is largely confined to proliferation centers (described later), where tumor cells receive critical cues from the microenvironment. Stromal cells in proliferation centers express a variety of factors that stimulate the activity of the transcription factors nuclear factor kappa B (NF-κB), which promotes cell survival, and MYC, which promotes cell growth. Other critical signals are generated by the B-cell receptor (membrane bound Ig), which triggers a signaling pathway that includes Bruton tyrosine kinase (BTK). The importance of BTK in B cells is emphasized by congenital X-linked agammaglobulinemia ( Chapter 6 ), a failure of B-cell development that is caused by loss-of-function mutations in the BTK gene. Notably, BTK inhibitors produce sustained therapeutic responses in a high fraction of CLL patients, indicating that CLL cells depend on B-cell receptor signaling and BTK activity for their growth and survival.

Morphology

Lymph nodes are diffusely effaced by predominantly small lymphocytes 6 to 12 µm in diameter with round to slightly irregular nuclei, condensed chromatin, and scant cytoplasm ( Fig. 13.7 ). Admixed are variable numbers of larger activated lymphocytes that often gather in loose aggregates referred to as proliferation centers that contain mitotically active cells. When present, proliferation centers are pathognomonic for CLL/SLL. The blood contains variable numbers of small round lymphocytes with scant cytoplasm ( Fig. 13.8 ). Some of these cells are usually disrupted in the process of making smears, producing so-called smudge cells. In almost all cases, the bone marrow, spleen, and liver ( Fig. 13.9 ) also are involved, albeit to widely varying degrees.

Clinical Features

Patients are often asymptomatic at diagnosis. When symptoms appear, they are nonspecific and include easy fatigability, weight loss, and anorexia. Generalized lymphadenopathy and hepatosplenomegaly are present in 50% to 60% of symptomatic patients. The leukocyte count is highly variable; leukopenia can be seen in individuals with SLL and marrow involvement, while counts in excess of 200,000/mm ³ are sometimes seen in CLL patients with heavy tumor burdens. A small monoclonal Ig “spike” is present in the blood of some patients. At the other end of the spectrum are asymptomatic patients with monoclonal B cells in their peripheral blood that number too few to merit the diagnosis of CLL. This condition, referred to a monoclonal lymphocytosis of uncertain significance, is considered to be a precursor lesion and progresses to symptomatic CLL at a rate of 1% per year.

CLL/SLL disrupts normal immune function through uncertain mechanisms. Hypogammaglobulinemia is common and contributes to an increased susceptibility to infection, particularly those caused by bacteria. Conversely, 10% to 15% of patients develop hemolytic anemia or thrombocytopenia due to autoantibodies made by nonneoplastic B cells.

The course and prognosis are extremely variable and depend primarily on the clinical stage. Overall median survival is 4 to 6 years but is more than 10 years in individuals with minimal tumor burden at diagnosis. Other variables that correlate with a worse outcome include (1) the presence of deletions of 11q and 17p (the latter involving TP53 ), (2) a lack of somatic hypermutation, (3) the expression of ZAP-70, a protein that augments signals produced by the Ig receptor, and (4) the presence of NOTCH1 mutations. Symptomatic patients are generally treated with “gentle” chemotherapy and immunotherapy with antibodies against proteins found on the surface of CLL/SLL cells, particularly CD20. Newly available, highly active targeted therapies include BTK inhibitors and BCL2 inhibitors; their impact on the disease course is still being determined.

Another factor that impacts patient survival is the tendency of CLL/SLL to transform to a more aggressive tumor. Most commonly this takes the form of a transformation to diffuse large B-cell lymphoma (DLBCL), so-called Richter syndrome (approximately 5% to 10% of patients). Richter syndrome is often heralded by the development of a rapidly enlarging mass within a lymph node or the spleen. It is often associated with acquisition of new abnormalities involving TP53 or MYC and is an ominous event, with most patients surviving less than 1 year.

Pathogenesis

Follicular lymphoma is strongly associated with chromosomal translocations involving BCL2. Its hallmark is a (14;18) translocation that juxtaposes the IGH locus on chromosome 14 and the BCL2 locus on chromosome 18. The t(14;18) is seen in up to 90% of follicular lymphomas and leads to overexpression of BCL2 (see Fig. 13.12 ). BCL2 antagonizes apoptosis ( Chapters 2 and 7 ) and promotes the survival of follicular lymphoma cells. Notably, while normal germinal centers contain numerous B cells undergoing apoptosis, follicular lymphoma is characteristically devoid of apoptotic cells. Deep sequencing of follicular lymphoma genomes have identified mutations in the KMT2D gene in about 90% of cases as well. KMT2D encodes a histone methyltransferase, suggesting that epigenetic abnormalities such as changes in the patterns of histone marks have an important role in this neoplasm.

Particularly early in the disease, follicular lymphoma cells growing in lymph nodes are found within a network of reactive follicular dendritic cells admixed with macrophages and T cells. Expression profiling studies have shown that differences in the genes expressed by these stromal cells are predictive of outcome, implying that the response of follicular lymphoma cells to therapy is influenced by the surrounding microenvironment.

Morphology

In most cases, a nodular or nodular and diffuse growth pattern is observed in involved lymph nodes ( Fig. 13.10A ). Two principal cell types are present in varying proportions: (1) small cells with irregular or cleaved nuclear contours and scant cytoplasm, referred to as centrocytes (small cleaved cells), and (2) larger cells with open nuclear chromatin, several nucleoli, and modest amounts of cytoplasm, referred to as centroblasts ( Fig. 13.10B ). In most follicular lymphomas, centrocytes are in the majority. Peripheral blood involvement sufficient to produce lymphocytosis (usually <20,000 cells/mm ³ ) is seen in about 10% of cases. Bone marrow involvement occurs in 85% of cases and characteristically takes the form of paratrabecular lymphoid aggregates. The splenic white pulp ( Fig. 13.11 ) and hepatic portal triads are also frequently involved.

Immunophenotype

The neoplastic cells closely resemble normal germinal center B cells, expressing CD19, CD20, CD10, surface Ig, and BCL6. Unlike CLL/SLL and mantle cell lymphoma, CD5 is not expressed. BCL2 is expressed in more than 90% of cases, in distinction to normal follicular center B cells, which are BCL2-negative ( Fig. 13.12 ).

Clinical Features

Follicular lymphoma tends to present with painless, generalized lymphadenopathy. Involvement of extranodal sites, such as the gastrointestinal tract, central nervous system, or testis, is relatively uncommon. Although incurable, it usually follows an indolent waxing and waning course. Survival (median, 7 to 9 years) is not improved by aggressive therapy; hence the usual approach is to palliate patients with low-dose chemotherapy or immunotherapy (e.g., anti-CD20 antibody) when they become symptomatic. Like CLL, it also is responsive to inhibitors of B-cell receptor signaling (e.g., BTK inhibitors) and inhibitors of BCL2.

Histologic transformation occurs in 30% to 50% of follicular lymphomas, most commonly to DLBCL. These transformation events are frequently associated with aberrations that increase the expression of MYC, which you will recall drives Warburg metabolism and rapid cell growth. Follicular lymphomas show evidence of ongoing somatic hypermutation, which may promote transformation by causing point mutations or chromosomal aberrations. The median survival is less than 1 year after transformation.

Pathogenesis

Genetic, gene expression profiling, and immunohistochemical studies indicate that DLBCL is molecularly heterogeneous. One frequent pathogenic event is dysregulation of BCL6, a DNA-binding zinc-finger transcriptional repressor that is required for the formation of normal germinal centers. About 30% of DLBCLs contain various translocations that have in common a breakpoint in BCL6 at chromosome 3q27. Acquired mutations in BCL6 promoter sequences that abrogate BCL6 autoregulation (an important negative-regulatory mechanism) are seen even more frequently. It is hypothesized that both types of lesions are inadvertent by-products of somatic hypermutation that result in overexpression of BCL6, which has several important consequences. BCL6 represses the expression of factors that normally serve to promote germinal center B-cell differentiation, growth arrest, and apoptosis, and each of these effects is believed to contribute to the development of DLBCL. Mutations similar to those found in BCL6 are also seen in multiple other oncogenes, including MYC, suggesting that somatic hypermutation in DLBCL cells is “mistargeted” to a wide variety of loci.

Another 10% to 20% of tumors are associated with the t(14;18) (discussed earlier under Follicular Lymphoma), which leads to the overexpression of the antiapoptotic protein BCL2. Tumors with BCL2 rearrangements usually lack BCL6 rearrangements, suggesting that these rearrangements define two distinct molecular classes of DLBCL. Some tumors with BCL2 rearrangements may arise from unrecognized underlying follicular lymphomas, which frequently transform to DLBCL. Roughly 5% of DLBCLs are associated with translocations involving MYC; these tumors may have a distinctive biology (discussed later under Burkitt lymphoma). Finally, sequencing of DLBCL genomes has identified frequent mutations in genes encoding histone acetyltransferases such as p300 and CREBP, proteins that regulate gene expression by modifying histones and altering chromatin structure.

Morphology

The common features are a relatively large cell size (usually four to five times the diameter of a small lymphocyte) and a diffuse pattern of growth ( Fig. 13.13 ). Other morphologic features show substantial variation. Most commonly, the tumor cells have a round or oval nucleus that appears vesicular due to margination of chromatin to the nuclear membrane, but large multilobated or irregular nuclei are prominent in some cases. Nucleoli may be two to three in number and located adjacent to the nuclear membrane or single and centrally placed. The cytoplasm is usually moderately abundant and may be pale or basophilic. More anaplastic tumors may contain multinucleated cells with large inclusion-like nucleoli that resemble Reed-Sternberg cells (the malignant cell of Hodgkin lymphoma).

Immunophenotype

These mature B-cell tumors express CD19 and CD20 and show variable expression of germinal center B-cell markers such as CD10 and BCL6. Most have surface Ig. High-level expression of both MYC and BCL2 proteins is seen in some cases and may predict more aggressive behavior.

Special Subtypes

Several subtypes of DLBCL are sufficiently distinctive to merit brief discussion.

• Immunodeficiency-associated large B-cell lymphoma occurs in the setting of T-cell immunodeficiency (e.g., in advanced HIV infection and in recipients of organ or HSC transplants). The neoplastic B cells are usually infected with EBV, which plays a critical pathogenic role. Restoration of T-cell immunity may lead to regression of these proliferations.

• Primary effusion lymphoma presents as a malignant pleural or ascitic effusion, mostly in patients with advanced HIV infection or in older adults. The tumor cells are often anaplastic in appearance and typically fail to express surface B- or T-cell markers, but have clonal IGH gene rearrangements. In all cases the tumor cells are infected with HHV-8, which appears to have a causal role.

Clinical Features

DLBCL typically presents as a rapidly enlarging mass at a nodal or extranodal site. It can arise virtually anywhere in the body. Waldeyer ring, the oropharyngeal lymphoid tissue that includes the tonsils and adenoids, is involved commonly. Primary or secondary involvement of the liver and spleen may take the form of large destructive masses ( Fig. 13.14 ). Extranodal sites include the gastrointestinal tract, skin, bone, brain, and other tissues. Bone marrow involvement is relatively uncommon and usually occurs late in the course. Rarely, a leukemic picture emerges.

DLBCLs are aggressive tumors that are rapidly fatal without treatment. With intensive combination chemotherapy, 60% to 80% of patients achieve a complete remission, and 40% to 50% are cured. Adjuvant therapy with anti-CD20 antibody improves both the initial response and the overall outcome. Individuals with limited disease fare better than those with widespread disease or bulky tumor masses. Expression profiling has identified several distinct molecular subtypes, including one resembling germinal center B cells and a second resembling activated post–germinal center B cells, each with differing clinical outcomes. DLBCLs with MYC translocations have a worse prognosis than those without and may be better treated with chemotherapy regimens that are now standard for Burkitt lymphoma. CAR T cells directed against the B-cell antigen CD19 are now available for the treatment of patients with relapsed refractory DLBCL.