Is characterized by the leukemic expansion of neoplastic post-thymic T-cell lymphocytes. More than 90% of the cases feature chromosomal abnormalities that involve the 14q32 or Xq28 loci associated with the increased cytoplasmic expression of TCL1A, TCL1B, or MTCP1. Immunophenotypically, the tumor cells are characterized by CD4 expression in 75% of the cases and positive expression of both CD4 and CD8 markers in 25% of the cases (8, 9). The WHO-HAEM5 incorporated diagnostic recommendations from the T-PLL international study group (TPLL-ISG) assembled in 2017 (10). The proposed guidelines indicate that at least 2 (out of 3) major criteria must be met to establish a diagnosis of T-PLL ( Table 2 ). The major criteria include defining T-PLL characteristics, including peripheral blood/bone marrow involvement, specific genetic aberrancies (TCL1A/B and MTCP), and T-cell clonality ( Table 2 ). Importantly, less than 10% of the cases will feature characteristic clinical features of T-PLL, yet genetic aberrancies of TCL1A, TCL1B, or MTCP are not identified. This group of patients is recognized as TCL1-family negative T-PLL. In these scenarios, the proposed diagnostic criteria indicate that at least one chromosomal abnormality specific to T-PLL or the involvement of T-PLL-specific sites (spleen, effusions, skin, and CNS) must be present to establish a definitive diagnosis of T-PLL (1 minor criteria, Table 2 ).

Is defined by a persistent (at least 6 months) increase in the number of circulating large granular lymphocytes (2-20 x 10⁹/L). About 40-50% of the patients will develop neutropenia, 30-40% anemias, and 10-25% splenomegaly. The updated WHO-HAEM5 incorporated novel molecular findings from two studies that established a correlation between the genomic landscape and clinical characteristics.

The first study (Sannikommu et al.) conducted a retrospective analysis of 224 patients with T-LGL (median follow-up of 36 months). It evaluated the clinical outcomes, therapy responses, and the mutational status of STAT3 ( 11). In this study, mutations in STAT3 were identified in 36% of the patients, and the most frequent change was Y640F-STAT3. The study demonstrated that patients with STAT3 mutations are more likely to feature neutropenia and anemias. However, no differences in overall survival were identified when patients were stratified according to the mutational status of STAT3.

The second (Barila et al.) is a retrospective study that evaluated 169 patients diagnosed with T-LGL between 1992 and 2018 (12). STAT3 mutations were identified in 60% of the patients with CD8+ T-LGL (n=65). However, mutations in STAT3 were not detected in patients with CD4+/CD8^neg/dim T-LGL. STAT5b mutations were detected in 34% of CD4+/CD8^neg/dim T-LGL patients and not in CD8+ T-LGL patients. Correlation with clinical characteristics showed that patients with STAT3 mutations were characterized by a higher frequency of neutropenia, anemia, and transfusion-dependent anemia. Multivariate analysis demonstrated that mutations in STAT3 were independently associated with reduced overall survival (267 months vs. not-reached).

The study by Barila et al. also included 36 patients with CLPD-NK (12). The clinical characteristics of CLPD-NK were like the T-LGL counterparts. Approximately 26% of the patients featured anemia, 48% neutropenia, and 13% splenomegaly. However, mutations in STAT3 were identified only in 6% of the patients, and STAT5b mutations were not detected. Expression of CD94 was identified in 94% of the CLPD-NK cases. In contrast, CD94 expression was detected in 14% of Tα/β T-LGL and 52% of Tγ/δ T-LGL cases. Due to the clinical similarities with T-LGL, the WHO-HAEM5 has renamed this entity NK-large granular lymphocytic leukemia (NK-LGL).

Is an aggressive T-cell neoplasm caused by infection of the human T-cell leukemia virus type 1 (HTLV-1). Therefore, it is predominantly diagnosed in HTLV-1 endemic areas including Japan, Central America, South America, and intertropical Africa (13). The neoplastic lymphocytes show variable morphology and range from small to large forms, with irregular nuclear contours, ‘flower-like cells,’ and occasionally vacuolated cytoplasm (14). CD3/CD4 positive, mature T-cells characterize the immunophenotype with co-expression of CD25, FoxP3, and lack of CD7 expression (14). The most common presentation (acute type, ~60%) and the chronic variant (~15% of cases) are characterized by leukemic involvement with hepatosplenomegaly. Predominantly nodal and cutaneous manifestations occur in the smoldering and lymphomatous variants (~25% of cases). Those two variants are more frequent in geographical areas where HTLV-1 is nonendemic, and the diagnosis is usually problematic because those subtypes can mimic cutaneous T-cell lymphomas or, more rarely, anaplastic large cell lymphoma (15). The prognosis is poor, regardless of the clinical presentation, and ranges from 1 year to 2 years after diagnosis. HTLV-1 serology testing is non-diagnostic of ATLL, and direct detection of viral transcripts in the neoplastic cells is critical for a definitive diagnosis in cases where the clinical and histopathological presentation is suggestive of different subtypes of T-cell lymphomas (15, 16).

The updated version of WHO-HAEM5 has incorporated novel genomic findings from two recent studies. The first study (Kogure et al.) identified recurrent loss-of-function mutations in the CIC-L/ATXN1 complex after performing whole-exome sequencing in 150 ATLL patients (17). The CIC-L gene encodes a transcriptional repressor that complexes with the ATXN1 protein. Mutations or structural variations in CIC-L and ATXN1 were mutually exclusive in ATLL patients, and in combination, alterations in the ATXN1 and CIC-L complex were identified in 53% of the cases (17). To support the oncogenic role of CIC-L/ATXN1 during the neoplastic expansion of ATLL, murine models demonstrated that conditional deletion of CIC-L in CD4+ T-cell lymphocytes was associated with the proliferation of CD4+CD25+CD127-FoxP3+ regulatory T-cells.

A second study evaluated the clinical outcomes and the genetic landscape of 463 ATLL patients (18). Patients with aggressive forms of ATLL displayed a higher number of mutations. The 3-year overall survival rate was 54% for patients with 0 to 1 mutation, 39% for those with 2-5 mutations, and 19% for those with more than 6 mutations (18). Multivariate analysis demonstrated that older age (more than 70 years), PRKCB mutations, and PD-L1 amplifications were independent prognostic factors for poor overall survival. Copy number amplifications of PD-L1 were more frequent in the aggressive group and were predictive of poor outcomes in patients with indolent and aggressive forms of the disease. Among the patients with indolent clinical behavior, IRF4 mutations, PD-L1 amplifications, and CDKN2A deletions predicted poor outcomes (18). Overall, these findings highlight the relevance of including genomic profiling for the classification and prognosis of ATLL.

Is a rare NK-cell neoplasm characterized by a fulminant clinical course with a median overall survival of 2 months or less after diagnosis. A leukemic presentation and occasional skin and CNS involvement characterize this entity. The WHO-HAEM5 included under this category the cases of NK/T cell lymphoma with an intravascular presentation that were previously considered a variant of extranodal NK/T-cell type lymphoma. In addition, novel clinical findings and mutational analysis are included under this category in the updated WHO-HAEM5.

A recent large retrospective evaluated the molecular profile and clinical features of 161 individuals diagnosed with ANKL in China (19). The peak incidence was 21 and 30 years old, with a median overall survival of 55 days. A group of the patients (16%) demonstrated a prolonged prodromal disease (subacute ANKL) that was characterized by fever, lymphocytosis, generalized lymphadenopathy, and hepatosplenomegaly (infectious mononucleosis-like symptoms). The prodromal phase has a median duration of 115 days, precedes the fulminant onset of ANKL, and the overall mean survival in this group of patients is 213 days. In contrast, patients without a prodromal phase (‘classic’ ANKL) have a median overall survival of 44 days. A targeted sequencing panel identified similar frequencies of mutations in the JAK-STAT pathway between the two groups. Importantly, mutations in TP53 were detected in 38% of the ‘classic’ ANKL patients, and no mutations in TP53 were identified in the subacute group (19).

A second study evaluated the genomic landscape of 10 ANKL patients utilizing whole-exome sequencing. The mutational spectrum of ANKL clustered separately from other related mature T-cell lymphomas with leukemic presentation (T-LGLL, T-PLL, and CPLD-NK) (20). The most recurrently mutated genes were DDX3X (29%) and STAT3 (21%), and mutations in TP53 were detected in a single patient. The most frequent mutations in ANKL were also seen in NK-T-cell lymphoma, nasal type (NKTCL); therefore, no specific mutations were identified in the ANKL that can help distinguish those cases from NKTCL (20).

Is morphologically characterized by superficial infiltrates composed of small lymphocytes with minimal cytological atypia that exhibit a Ki-67 proliferation index that is less than 10% (31). This entity is characterized by chronic and relapsing gastrointestinal symptoms, including diarrhea, dyspepsia, and vomiting (32, 33). In some cases, disease progression with an aggressive clinical course has been reported (34, 35). Therefore, the term ‘lymphoproliferative’ has been replaced by ‘lymphoma’ to highlight that this entity represents a clonal disease with a risk of transformation into more aggressive forms.

Has been renamed indolent NK-cell lymphoproliferative disorder of the GI tract (iNKLPD). This entity is characterized by non-specific gastrointestinal symptoms, including abdominal pain, constipation, and reflux, with a chronic/relapsing course (36). The tumor cells feature mild cytological atypia, and the Ki-67 proliferation index is usually in the 25%-50% range; in contrast to ENKTCL, these tumors are negative for EBV expression. Additional case series reports have confirmed the indolent clinical course, with no reported cases of progression to more advanced stages (37). A recent study in 7 patients identified somatic mutations in JAK3 in 30% of the patients; additional mutations in other genes, including IFG1R and AURKB, were also detected at lower frequencies (38). The same study demonstrated increased expression of phosphorylated STAT5 in 100% of cases (n = 7) (38), supporting the idea that this process constitutes a neoplastic lymphoproliferative disease.

ALCL is the third most common nodal T-cell lymphoma and accounts for approximately 11-13% of peripheral T-cell lymphomas (39, 40). Morphologically is characterized by large anaplastic cells, ‘hallmark cells,’ organized in a cohesive pattern, with uniform and strong expression of CD30 in more than 80% of the tumor cells (41) ( Figure 4 ). Translocations involving the tyrosine kinase ALK are present in approximately 50% of ALCL cases. The uncontrolled activation of the ALK-kinase defines a phosphoproteomic and transcriptional signature that drives the oncogenic program of ALK+ ALCL cases (42–44), and ALK+ ALCL constitutes a specified entity. ALK-negative ALCL comprises a heterogenous group and is characterized by inferior clinical outcomes compared to ALK+ ALCL. Specific genomic rearrangements within this group, including DUSP22, and TP63 fusions, are associated with differential clinical outcomes (45–48). The WHO-HAEM5 has included novel genomic findings from two studies that will help understand the biology of ALCL.

Hapgood et al. (48) correlated the outcomes and molecular features of 62 ALK-negative ALCL patients. The findings demonstrated inferior clinical outcomes in patients with DUSP22 rearrangements (n = 12, 44% five-year OS) in comparison to ≥80% 5-year overall survival (OS) that was described in the previous case series (47). These findings indicate that additional molecular mechanisms can modify the outcomes of these patients. Consistent with this, a recent cohort study in 82 ALCL patients with systemic ALCL (ALK+ and ALK-) identified that mutations in TP53 and STAT3 are associated with a worse prognosis independent of ALK status (49).

The second study demonstrated the presence of specific super-enhancer (SE) regions enriched with BATF3 sites in ALCL cell lines and patient samples. The findings further revealed that the transcriptional activity of BATF3 can mediate the expression of IL2R. Importantly, increased expression of IL2R was predominantly detected in ALCL cases compared to different peripheral T-cell lymphomas, including AITL and PTCL-NOS. The findings indicate that chromatin remodeling may play a critical role during the growth of these groups of tumors and that IL2-dependent signaling plays a pivotal role in substituting T-cell receptor signaling in ALCL tumors (50).

Finally, the WHO-HAEM5 incorporated findings from a recent study by Fitzpatrick et al. that identified JAK2 fusions in 6% of systemic CD30+ ALK-negative T-cell lymphomas (n=97) (51). Among the cases with JAK2 translocations, three fulfilled the diagnostic criteria of ALK-negative ALCL, and the other three were diagnosed as CD30+ PTCL-NOS. All the cases with JAK2 translocations featured at least a proportion of ALCL-like large anaplastic tumor cells, in addition to Reed-Sternberg (RS)-like cells. Co-expression of CD30 and CD15 was present in the large anaplastic component in 80% of the cases. In all instances, the tumor cells were negative for PAX-5, and T-cell clonality was established. Due to the limited number of cases, a comparison of differential clinical outcomes was not possible (51). The presence of RS-like cells with co-expression of CD30 and CD15 can be a diagnostic pitfall, and recognition of this subset of cases is critical during diagnosis.

Peripheral T-cell lymphomas derived from T-follicular helper (T-FH) lymphocytes encompass a spectrum of aggressive neoplasms with poor clinical outcomes (61). Two entities in this group are characterized by specific morphological features: angioimmunoblastic T-cell lymphoma (AITL) and Follicular T-cell lymphoma (F-TH). A third group comprises neoplastic expansions of T-follicular helper T-cell lymphocytes that do not fit the characteristic morphological patterns of AITL and T-FH lymphoma. This group was defined in the latest WHO-HAEMR4 as Nodal Peripheral T-cell lymphoma with TFH phenotype (PTCL-TFH).

AITL is the second most common type of nodal T-cell lymphoma and accounts for 15-20% of peripheral T-cell lymphomas. Morphologically is characterized by effacement of the nodal architecture, with an expansion of arborizing post-endothelial venules with clusters of neoplastic lymphocytes with clear cytoplasm ( Figure 5 ). These areas characteristically feature an expansion of follicular dendritic cell meshworks. The diagnosis can be difficult because the tumor cells represent a minority of the infiltrates, and the tumor microenvironment features positive RS-like cells for EBV with eosinophilic infiltrates (62).

This group remains the most prevalent in the United States and constitutes a ‘waste-basket’ category when other disease entities are excluded from the current classification schemes. Recent study series that correlated clinicopathological features with genomic/molecular data has helped to identify distinct groups that are no longer included within this classification, such as Nodal EBV+ T/NK-cell lymphomas and Nodal T-follicular helper lymphomas, NOS.

Previous studies demonstrated that the expression of the transcription factor GATA-3 is enriched in a subset of PTCL-NOS cases with characteristic transcriptional profiles, worse overall survival, and resistance to chemotherapy (71–74). A second, albeit more heterogeneous subtype, highly expresses the transcription factor TBX21 and is similarly enriched for its gene targets. An immunohistochemistry algorithm that includes CD183, CCR4, GATA-3, and TBX21 has been proposed to identify these cases (75) ( Figure 7 ). The upregulated expression of GATA-3 is secondary to the engagement of T-cell receptors in the neoplastic cells, and this is likely secondary to specific interactions with the tumor microenvironment (76–78). Due to insufficient clinicopathological and prognostic findings, the current WHO-HAEM5 did not recognize this group of tumors as a specific subtype of PTCL.

ENKTL is an aggressive type of lymphoproliferative disorder predominantly derived from NK cells, with a minority of cases from cytotoxic T-cell origin. Approximately 80% of the cases are localized in the nasal cavity or nasopharynx (85). The remaining 20% of the cases will involve other sites that include skin, testicles, and salivary glands. Morphologically the neoplastic infiltrate can show a broad spectrum of appearances ranging from small cells with minimal atypia to medium to large pleomorphic cells with marked cellular atypia (86, 87) ( Figure 8 ). Necrosis is observed in approximately 60%-80% of the cases, and angiocentricity is present in nearly 50% of the cases (85, 86, 88). Virtually all the cases are EBV+ in more than 80% of the tumor cells (85, 87, 88). An NK-cell immunophenotype is the most frequently observed and is characterized by negative expression of surface CD3, with positive expression of cytoplasmic CD3ε, CD56, and the cytotoxic markers TIA-1 and granzyme B. Approximately 20% of true NK-cell origin tumors (germline TCRγ) will feature negative expression of CD56, however, expression of cytotoxic markers is always present (87–89). Cases with a T-cell origin defined by monoclonal TCRγ rearrangement can account for 20%-40%, and approximately 40% of those will lack CD56 expression (85, 89). However, EBV negative cases with negative expression of CD56 should be designated as PTCL-NOS (87). In recognition that a subset of these cases involves extra-nodal sites other than the nasal cavity, the WHO-HAEM5 has removed the term ‘nasal-type’ from this designation. In addition, the WHO-HAEM5 mentioned novel genomic findings that highlight the relevance of STAT3 mutations in ENKTL and the therapeutic role of PD-L1.

A recent study by Song et al. conducted targeted gene capture sequencing in 171 PTCLs, and JAK/STAT mutations were identified in 78% of the ENKTL cases (n=109) (90). Mutations in STAT3 were predominantly detected in ENKTL cases (21%), in contrast to ALCL (3.7%) and PTCL-NOS cases (3.8%). The mutations identified in STAT3 were associated with increased transcriptional activity and higher expression of PD-L1 in-vitro. However, the evaluation of primary ENKTL samples showed that 93% of the cases (n=30) featured increased expression of PD-L1, suggesting that additional molecular mechanisms are involved in PD-L1 expression (90). Consistent with this, Bi et al. demonstrated that the expression of the EBV latent membrane protein 1 (LMP1) promotes PD-L1 expression in ENKTL cell lines in-vitro (91).

Finally, the therapeutic potential of PD-L1 in ENKTL has been evaluated in a recent phase 2 clinical trial that tested the efficacy of the IgG1-PD-L1 antibody avelumab as a single agent in refractory/relapsed (R/R) ENKTL (92). This study by Jin et al. demonstrated a 24% complete response rate in R/R patients that received avelumab (92). The patient responses to avelumab were associated with higher expression of PD-L1 in the tumor cells. However, response rates were not related to the levels of serum soluble PD-L1 (92).

This novel designation encompasses EBV+ NK/T cell lymphomas with a predominant nodal presentation. These tumors are characteristically diagnosed at an advanced stage and feature dismal clinical outcomes with overall survival of 1.5-3.5 months after diagnosis (93–95). This new entity differs from ENKTCL with secondary nodal involvement, as the former will primarily be present at extranodal sites. Cases of chronic active EBV infection of T-cell or NK-cell type (T/NK- type CAEBV) are also excluded from this category, as those do not feature morphological features consistent with a lymphoproliferative neoplasm. Finally, the presence of leukemic involvement by neoplastic cells with an NK-cell immunophenotype will be best classified as ANKL.

Other nodal T-cell lymphomas can feature a subset of tumor cells positive for EBV. Therefore, the minimal threshold for the number of EBV+ neoplastic cells varies among the published case series and ranges between 40%-50% (93–95). Immunophenotypically, approximately 60% of the cases will show tumor cells with a T-cell immunophenotype that are positive for CD8 and TβF1. The tumor cells will be less frequently positive for CD4 (93–95). Tumor cells with a characteristic NK-cell immunophenotype have been described. However, those account for less than 10% of the cases described (93, 94, 96).

This group of neoplasms features gene-expression profiles distinct from ENKTL with frequent loss of 14q11.2 and upregulation of PD-L1 (CD174) (93). Functional analysis of the gene-expression profiles demonstrates enrichment for NF-κB signaling (97). Consistent with this and in contrast to PTCL-NOS cases, predominant expression of BIRC3 and p50 is detected (97). Frequent mutations in TET2 (64%, n=14), followed by PIK3CD (33%, n=14) and STAT3 (19%, n=14) (97) characterize the genomic landscape of EBV+ NTNKCL.