Generation of competent bone marrow-derived antigen presenting cells from the deer mouse (Peromyscus maniculatus)

Abstract

Background: Human infections with Sin Nombre virus (SNV) and related New World hantaviruses often lead to hantavirus cardiopulmonary syndrome (HCPS), a sometimes fatal illness. Lungs of patients who die from HCPS exhibit cytokine-producing mononuclear infiltrates and pronounced pulmonary inflammation. Deer mice (Peromyscus maniculatus) are the principal natural hosts of SNV, in which the virus establishes life-long persistence without conspicuous pathology. Little is known about the mechanisms SNV employs to evade the immune response of deer mice, and experimental examination of this question has been difficult because of a lack of methodologies for examining such responses during infection. One such deficiency is our inability to characterize T cell responses because susceptible syngeneic deer mice are not available.

Results: To solve this problem, we have developed an in vitro method of expanding and generating competent antigen presenting cells (APC) from deer mouse bone marrow using commercially-available house mouse (Mus musculus) granulocyte-macrophage colony stimulating factor. These cells are capable of processing and presenting soluble protein to antigen-specific autologous helper T cells in vitro. Inclusion of antigen-specific deer mouse antibody augments T cell stimulation, presumably through Fc receptor-mediated endocytosis.

Conclusions: The use of these APC has allowed us to dramatically expand deer mouse helper T cells in culture and should permit extensive characterization of T cell epitopes. Considering the evolutionary divergence between deer mice and house mice, it is probable that this method will be useful to other investigators using unconventional models of rodent-borne diseases.

Figure 1

Amino acid alignment of deer mouse (DM), cotton rat (CR), house mouse (HM) and human (HU) GM-CSF. Polypeptides were aligned with the clustal algorithm in Macvector. Conserved (light shading) and identical (dark shading) amino acids are enclosed in boxes. The 25-residue signal peptide is enclosed in box A. Helix A, which binds to the β chain subunit of the GM-CSF receptor, is enclosed in box B. Deer mouse and house mouse GM-CSF share 13 of 15 identical residues in this domain.

Figure 2

Morphologic characteristics of deer mouse bone marrow-derived APC. Day 14 bone marrow cells cultured in GM-CSF were processed by cytospin and stained with Wright's stain. The cells exhibited conspicuous cytoplasmic vesicles and small processes (A). Cells collected on day 12 and incubated for 48 hours with 20 ng/ml of hmTNF displayed less conspicuous cytoplasmic vesicles (B).

Figure 3

Proliferation of deer mouse cells to recombinant cytokines. (A) After 8 days of incubation with GM-CSF, deer mouse bone marrow cells were washed and then cultured with dilutions of GM-CSF in duplicate for 48 hours, then proliferation assessed by MTS assay. The data are representative of four deer mice. (B) To assess proliferative capacity of deer mouse T cells to human IL-2, splenocytes were cultured with a suboptimal dose of PHA (2 μg/ml) and dilutions of recombinant human IL-2 in duplicate for 48 hours, and proliferation assessed by MTS assay. The data are representative of two deer mice.

Figure 4

RT-PCR of TCRβC and MHC class II I-Eβ in deer mouse cells. Total RNA was extracted from T cells and bone marrow-derived APC. Expression of the constant β chain of the TCR by the T cells and class II I-Eβ chain by the APC were detected by RT-PCR. β-Actin primers were used as controls for each sample.

Figure 5

BM-APC stimulate helper T cell proliferation. Deer mice were immunized with 20 μg of KLH subcutaneously and 10 days later the lymph nodes, bone marrow and splenocytes were retrieved for expansion of helper T cells and BM-APC. After expansion of these cells in culture, proliferation assays were performed comparing mitomycin-C-treated autologous splenocytes (SC) and BM-APC (BMC) for their capacity to stimulate T cells. In each instance, the BM-APC were more effective at stimulating T cell responses. The data are of two deer mice (DM212 and DM213).

Figure 6

Antigen-specific antibody augments BM-APC-induced T cell proliferation. T cell proliferation responses from deer mice 223 and 224 were assessed as described in Figure 5 using BM-APC. KLH-specific antiserum or normal deer mouse serum were diluted 1:2,000 in DMM-5 and incubated with dilutions of KLH for 1 hour in 96-well plates at room temperature. BM-APC and T cells were added to the wells and incubated 72 hours, and proliferation was assessed by MTS assay.

Figure 7

Schematic overview of the process for culturing autologous deer mouse BM-APC and T cells. Ten days post immunization, lymph nodes, spleens and bone marrow are harvested from euthanized deer mice. The lymph node cells are cultured with antigen for four days, while the splenocytes and bone marrow cells are frozen at -70°C. On day 4, the blasting lymph node T cells are recovered and cultured with fresh antigen and thawed splenocytes (without mitomycin-C treatment). Simultaneously, bone marrow cells are thawed and cultured with GM-CSF. Expansion of the T cells is performed with huIL-2 and the BM-APC with GM-CSF for 14 days. These cells are then used for proliferation experiments.