University of Virginia Health System

Acute Lymphoblastic Leukemia (ALL)
L1 subtype

ALL L1- Bone marrow aspirate smear, Wright-Giemsa stain, 1000x

Description:

The blasts of the L1 subtype are small, often no larger than normal small peripheral blood lymphocytes. The nuclei are usually round, but may be slightly oval and indented, with light staining and evenly dispersed nuclear chromatin. Nucleoli frequently are not visible. The cytoplasm is scant and grey to light blue in color. Cytoplasmic granules are usually absent, but may be seen in some cases, especially in those that are Philadelphia chromosome positive. The cell surface may be smooth or show fine membrane projections.

Pathobiology:

In L1 and L2 ALLs, the proliferating cells are B cells and represent various stages of antigen-independent differentiation in the bone marrow. In children more than 85% of ALLs are of the L1 subtype, and essentially all are of B cell origin. In the L3 subtype, essentially 100% are of B cell origin. In 10%-15% of ALL cases the cells are of T cell lineage, and again, represent T cells at various stages of antigen-independent proliferation in the thymus. T cell ALLs tend to have more of an L2 morphology, and frequently demonstrate the characteristic convoluted nuclear appearance. In adults, the L2 subtype represents the majority of cases. On a paraffin fixed bone marrow specimen, reticulin fibrosis is seen in a majority of cases of ALL. The morphology of the malignant lymphoblasts in B cell and T cell lymphomas is the same as in the leukemias. Most lymphoblastic lymphomas are of T cell lineage, and frequently present in the thymus. 

Cytochemistry:

The blasts may be periodic-acid-Schiff (PAS) positive, but are myeloperoxidase (MPO) and Sudan black-B negative. Non-specific esterase (NSE) positivity is seen in some cases, and the acid phosphatase stain is often focally positive in T cell ALL.

Immunophenotypes:

• Although the immunophenotype varies, most are cCD79a+, CD19+, CD10+, CD34+, and TdT+, but do not express surface immunoglobulin (sIg). Cytoplasmic mu (μ) heavy chains are found in the pre-B cell subtype. The absence of sIg indicates that ALL of the L1 and L2 subtypes represents proliferation of early B cells prior to the immature B cell stage. The finding of sIg in some cases does not exclude a precursor B cell origin, as does the finding of myeloid markers, CD13 and 33. Based on the immunophenotypic profile alone 4 subsets can be identified:
  • Pro-B-ALL: CD34+, TdT+, cCD79+ , and CD19+ (Prognosis poor). 
  • Common-ALL: Above markers positive plus CD10 positivity. This subset represents 2/3 of all childhood cases and is a good prognostic subgroup.
  • Pre-B-ALL: characterized by presence of cytoplasmic μ heavy chains.
  • B-ALL: CD19+, sIg+, CD10±

• As with B cell ALL, the immunophenotype varies, but most are cytoplasmic CD3+ (cCD3+), CD7+, TdTt+, and CD34+. The expression of other T cell markers such as CD3, CD2, CD1a (cortical thymocyte marker), CD4, and CD8 is more variable. Some have a double negative (CD4-, CD8-) phenotype, and some a double positive (CD4+, CD8+) one. Clonality can be documented by molecular genetic analysis. Three subsets are recognized based on immunophenotypic profiles:
  • Early T-ALL: cCd3+, CD7+, CD5± , CD2±, CD1a-
  • Cortical T-ALL: CD1a+, cCD3+, CD7+, sCD3±
  • Mature T-ALL: sCD3+, CD1a-

Cytogenetics:

• Favorable cytogenetics include hyperdiploidy (>50 chromosomes) and some translocations, e.g. t(10;14) and t(12;21).
• Unfavorable cytogenetics include most of the rest, especially hypodiploidy, and translocations such as t(9;22) (Philadelphia chromosome), and those involving the 11q23 gene locus. The incidence of the various cytogenetic abnormalities differs considerably between children and adults. For example, hyperdiploidy is found in 25% of childhood cases versus only 7% in adults, and the t(9;22) is found in only 3% of children versus 25% of adults.

• Most of the cytogenetic abnormalities identified to date involve translocations at the alpha and delta T cell receptor (TCR) loci on Chromosome 14 or the beta and gamma TCR loci on Chromosome 7. Several partner genes are involved (e.g. TAL1 at 1p32, HOX11 at 10q24) 

Note: The morphologic, immunophenotypic, cytogenetic, and cytochemical features used to separate the various subtypes of acute leukemia are not absolute. Hybrid cases occur, and include not only mixed lineage cases, but also unusual profiles within the same lineage.

Differential Diagnosis:

• Micromegakaryocytic acute leukemia (M7)
  • Fine cytoplasmic projections are much more prominent in M7 AML, and cytoplasmic granules are usually present. The size of the blasts often are much more heterogeneous in M7 AML with some blasts being quite large, resembling normal megakaryoblasts.
• Microblastic AML
  • Morphologically, cannot distinguish this variant of AML which includes the M0 and M1 subtypes from ALL, L1 or L2 subtypes. The finding of Auer rods establishes the diagnosis of M1 AML. Immunophenotypic, cytochemical and molecular analyses are necessary to establish diagnosis in most cases.
• Hematogones
  • These are normal lymphoid progenitor cells that are seen in a normal bone marrow, and are usually evenly distributed. They are increased in numbers in young children and in reactive states such as bone marrow recovery following chemotherapy. They are the same size as L1 lymphoblasts with similar nuclei except for more clumping of the nuclear chromatin, and nucleoli usually are not visible. The immunophenotype is similar to B cell ALL, L1 subtype, except for the coexpression of more mature B cell markers, (e.g. sIg), and they do not demonstrate clonality.
• Reactive lymphocytosis
  • From a morphologic standpoint, the heterogeneity of the cells in reactive lymphocytosis (e.g. infectious mononucleosis) is an important diagnostic feature. Depending on the duration of the infection at the time of examination of a peripheral blood smear, cells with morphologic features of immunoblasts, plasmacytoid lymphocytes, plasma cells and small lymphocytes may be seen. A very characteristic feature of reactive lymphocytosis of B or T cell lineage is the abundance of basophilic (blue) cytoplasm especially in the more immature cells.

Prognosis:

At least 80% or more of children with ALL are cured compared to less than 40% in adults. In children between the ages of 1-10 years, especially with the common B cell ALL subtype, the cure rate approaches 95%. Most of the data on adults comes from cohorts in which the lower age range is 15 years. The outcome in adults above the age of 30 years at diagnosis is worse, and above the age of 40 years, much worse. Outcome in T cell ALL versus B cell ALL is worse in children, but better in adults.

General References:

• Jaffe ES, Harris NL, Stein H, Vardiman JW, eds. Pathology and genetics of tumours of hematopoietic and lymphoid tissues. Vol. 3 of World Health Organization classification of tumors. Lyon, France: IARC Press, 2001
• Kersey, JH. Fifty years of studies of the biology and therapy of childhood leukemia. Blood 90:4243, 1997
• Pui CH, Evans WE. Treatment of acute lymphoblastic leukemia. N Engl J Med 354:166, 2006
• Pui CH, Relling MV, Downing JR. Acute lymphoblastic leukemia. N Engl J Med 350: 1535, 2004
• Ludwig WD, Raghavachar A, Thiel E. Immunophenotype classification of acute lymphoblastic leukemia. Baillierre Clm Haematol 7:235, 1994
• Kantarjian HM, Hoelzer D, Larson RA, eds: Advances in the treatment of adult acute lymphocytic leukemia, Part I and Part II. Hematol/Oncol Clin N Am vol 14 (6), 2000 and vol 15 (1), 2001
• Albritton K, Douer D, Gaynor PS, et al, eds. Treatment of acute lymphoblastic leukemia in the adolescent and young adult population. Am J Hematol/Oncol vol 6, No 4, Suppl 5, 2007
• Bennett JM, Catovsky D, Daniel MT, et al: French-American-British (FAB) Cooperative Group: The morphological classification of acute lymphoblastic leukemia- concordance among observers and clinical correlations. Br J Haematol 47:553, 1981

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