A novel adoptive transfer model of chronic lymphocytic leukemia suggests a key role for T lymphocytes in the disease

Abstract

Chronic lymphocytic leukemia (CLL) is an incurable adult disease of unknown etiology. Understanding the biology of CLL cells, particularly cell maturation and growth in vivo, has been impeded by lack of a reproducible adoptive transfer model. We report a simple, reproducible system in which primary CLL cells proliferate in nonobese diabetes/severe combined immunodeficiency/γc(null) mice under the influence of activated CLL-derived T lymphocytes. By co-transferring autologous T lymphocytes, activated in vivo by alloantigens, the survival and growth of primary CFSE-labeled CLL cells in vivo is achieved and quantified. Using this approach, we have identified key roles for CD4(+) T cells in CLL expansion, a direct link between CD38 expression by leukemic B cells and their activation, and support for CLL cells preferentially proliferating in secondary lymphoid tissues. The model should simplify analyzing kinetics of CLL cells in vivo, deciphering involvement of nonleukemic elements and nongenetic factors promoting CLL cell growth, identifying and characterizing potential leukemic stem cells, and permitting preclinical studies of novel therapeutics. Because autologous activated T lymphocytes are 2-edged swords, generating unwanted graph-versus-host and possibly autologous antitumor reactions, the model may also facilitate analyses of T-cell populations involved in immune surveillance relevant to hematopoietic transplantation and tumor cytoxicity.

Figure 1

CLL proliferation in NSG mice. (A) Retro-orbital sinus bleeding was done at 2-week intervals. CLL cells only proliferate in presence of human hematopoietic elements. Representative example of 9 CLL samples, with a minimum of 29 mice per group (supplemental Table 2). Histograms only show CFSE⁺ cells. (B) hMSCs are not necessary for CLL proliferation because virtually identical CFSE dilution patterns are seen in the presence or absence of transferred hMSCs. Representative example of 4 CLL samples with a minimum of 10 mice per group (supplemental Table 2) is shown. (C) Early after CLL transfer, most B cells in PB are CFSE⁻, deriving from transplanted hCD34⁺ cells (supplemental Figure 2). (D) Late after CLL transfer, a relatively selective loss of CFSE⁻ B cells occurs; remaining B cells are mostly CFSE⁺ and of leukemic origin (supplemental Figure 2).

Figure 2

T-cell expansion influences propagation of CLL cells. (A) Mean numbers of circulating T cells at 2-week intervals. CLL cells proliferate only when circulating T cells exceed a threshold (solid line); above this, additional T cells do not influence extent of CLL cell proliferation. For some cases, T-cell numbers exceed the threshold early (time point 1); for others, it takes much longer; and some the threshold is never exceeded. (B) CLL 753 as an example indicating that CLL proliferation only occurs when the threshold number is exceeded, at time point 2 (see also Figure 5D).

Figure 3

Growth of CLL cells in NSG mice is T-cell dependent. (A) Simultaneous analysis of autologous T-cell and CLL-cell proliferation in vivo. Because PBMCs containing both T and B cells were labeled before transfer, the dilution of CFSE fluorescence, as an indicator of cell division, could be measured at the same time. Three representative examples are provided: CLL412 at 2 weeks after transfer and CLL321 and 373 at 4 weeks after transfer. (B) Eliminating T cells with an anti–human CD3 mAb inhibits CLL cell growth (2 representative examples from 15 CLL samples with 118 mice, minimum 55 per group). Negligible numbers of T cells were detected in the blood by immunofluorescence at the time of cell transfer/mAb treatment. (C) Eliminating CD4⁺ T cells aborts CLL proliferation and impairs CD8⁺ cell growth; eliminating CD8⁺ T cells does not impair CLL cell proliferation (representative example from 6 CLL samples, 60 mice, minimum 10 per group).

Figure 4

Coinfusion of mature CD14⁺ or CD19⁺ cells and CLL cells obviate necessity for preconditioning with hCD34⁺ cells. (A) Two examples shown (7 CLL samples, in 37 mice with at least 10 per group). See “Cotransfer of mature allogenic APCs supports CLL proliferation” for experimental details. (B) CLL 1235 as an example of preconditioning with hCD34⁺ cells or CD14⁺ cells from the same CB sample equally supporting CLL growth (3 CLL samples, minimum 6 mice per group).

Figure 5

Expression of CD38. (A) The number of CD38⁺ cells in murine blood relates to CLL cell proliferation. At the first time point, CLL cells do not divide and the number of CD38⁺ leukemic cells is low. CD38-expressing CLL cells reach maximal levels at the second time point when CLL cells have divided, approximating that in donor patient's blood (solid line). (B) The percent of CD38-expressing cells in mouse blood correlates closely with that in patient's blood (10 CLL samples, 42 mice). Asterisks indicate experiments in which hCD34⁺ cells were administered. Note that the levels of CD38-expressing cells are similar between murine and human blood regardless of the system (hCD34⁺ cells or mature alloAPCs) used to permit CLL cell expansion in vivo. (C) Note that the activation states of CLL cells differ on the basis of anatomic location. Although there is considerable leukemic cell proliferation occurring in the spleen and 24.2% cells express CD38, there is negligible division occurring in PB, peritoneum, and BM. Data collected at death or time of sacrifice. (D) CLL (CD19⁺CD5⁺CFSE⁺) cells in the spleen far outnumber those in BM.

Figure 6

Localization and characterization of CLL cells in NSG spleen. CD20⁺ cells with monotypic IGH (μ) and immunoglobulin light (κ) chains form follicular structures in the spleen. These cells are also ROR1⁺. Note the intimate relationship between the follicular structures and blood vessels.

Figure 7

T cells localize preferentially in follicular structures found in the spleen. Spleen tissue with follicular structures stained with anti-CD20 (green) and anti-CD3 (red) mAbs. Images were taken with an Olympus IX70 confocal microscope, at 25°C, with the use of UPlanAPO 10×/0.40 objective. Antibodies were labeled with DyLight 488 and 649 (Jackson ImmunoResearch Laboratories Inc) excited with one 30-mW argon laser at 488 nm and one 5-mW helium-neon laser exciting at 633 nm. Samples were mounted in SlowFade Gold antifade reagent and data acquired with proprietary image acquisition software (Olympus).