A New Dynamic Model of CD8?T Effector Cell Responses via
CD4?T Helper-Antigen-Presenting Cells1
Jim Xiang,2Hui Huang, and Yongqing Liu
A long-standing paradox in cellular immunology has been the conditional requirement for CD4?Th cells in priming of CD8?CTL
responses. We propose a new dynamic model of CD4?Th cells in priming of Th-dependent CD8?CTL responses. We demonstrate
that OT II CD4?T cells activated by OVA-pulsed dendritic cells (DCOVA) are Th1 phenotype. They acquire the immune synapse-
composed MHC II/OVAII peptide complexes and costimulatory molecules (CD54 and CD80) as well as the bystander MHC class
I/OVAI peptide complexes from the DCOVAby DCOVAstimulation and thus also the potential to act themselves as APCs. These
CD4?Th-APCs stimulate naive OT I CD8?T cell proliferation through signal 1 (MHC I/OVAI/TCR) and signal 2 (e.g., CD54/
LFA-1 and CD80/CD28) interactions and IL-2 help. In vivo, they stimulate CD8?T cell proliferation and differentiation into CTLs
and induce effective OVA-specific antitumor immunity. Taken together, this study demonstrates that CD4?Th cells carrying
acquired DC Ag-presenting machinery can, by themselves, efficiently stimulate CTL responses. These results have substantial
implications for research in antitumor and other aspects of immunity. The Journal of Immunology, 2005, 174: 7497–7505.
priming (1). Such help was originally thought to be mediated by
CD4?T cell IL-2 acting at short range to promote CD8?T cell
Two models of CD4?T help for CD8?CTL responses have
been proposed previously, including the passive model of three-
cell interaction (3, 4) and the dynamic model of sequential two-cell
interactions by APCs (5). The three-cell model suggested that ac-
tivated CD4?T cells and naive CD8?T cells must interact si-
multaneously with a common APC and that the CD4?Th cells
provide CD8?T cell help via expression of IL-2 (Fig. 1A). The
conundrum, however, is how a rare Ag-specific CD4?Th cell and
an equally rare Ag-specific CD8?T cell (typically 1 in 105–106T
cells) would simultaneously find the same Ag peptide-carrying
APC in an unprimed animal (6). Instead, Ridge et al. (5) have
proposed a dynamic model of two sequential interactions, in which
an APC first offers costimulatory signals to a CD4?Th cell and
then to a CD8?CTL cell (Fig. 1B). According to this model, the
APC-stimulated CD4?Th cells must first reciprocally counter-
stimulate the APCs (through CD40 (CD40L)3signaling) such that
this newly “conditioned” APC can then directly costimulate CD8?
CTLs. Support for this model comprises evidence that Ag-specific
eneration of effective CTL responses to minor histocom-
patibility or tumor Ags not associated with danger sig-
nals often requires help from CD4?Th cells via cross-
CTL responses can be induced by vaccination with either large
numbers of APCs activated in vitro through CD40 signaling or, in
either MHC class II knockout (KO) or CD4?T cell-depleted mice,
by high level activation of APCs in vivo with anti-CD40 Ab (5,
7–9). Although this model provides a possible explanation for the
conditional nature of T cell help for CTL responses, the experi-
mental conditions used in the above studies may well not accu-
rately model the physiology of Th cell-dependent immune re-
sponses in vivo. In addition, a scarcity caveat remains (10), in that
very small numbers of Ag-bearing APCs (11) must first activate
and be conditioned by the rare naive Ag-specific CD4?Th cells,
and then find and activate in turn equally rare naive Ag-specific
CD8?CTLs. In addition, this model does not explain how Th
cells’ IL-2 would be precisely targeted to Ag-specific CD8?Ag-
specific CTLs. Furthermore, the life span of an activated dendritic
cell (DC) in the T cell zone of a lymph node is ?48 h (11–13),
possibly due to CD4?T cell killing of the cognate APCs (14–15),
whereas the Ag-specific CTL response is first detected at about day
5 in the lymph nodes (11, 16). Thus, this dynamic model also does
not explain compellingly the temporal gap between Ag presenta-
tion and the acquisition of CTL effector function in vivo.
It is recognized that stimulation of T cells by APCs involves at
least two signaling events: one elicited by TCR recognition of
peptide-MHC complexes and the other by costimulatory molecule
signaling (e.g., T cell CD28/APC CD80) (17). A consequence of
such Ag-specific T cell-APC interactions is the formation of an
immunological synapse, comprising a central cluster of TCR-
MHC-peptide complexes and CD28-CD80 interactions surrounded
by rings of engaged accessory molecules (e.g., complexed LFA-
1-CD54) (18, 19). One important feature of synapse physiology is
that APC-derived surface molecules are transferred to the Th cells
during the course of their TCR internalization followed by recy-
cling (20, 21). In theory, if tumor peptide-presenting DCs do trans-
fer a complete set of Ag-presenting molecules (e.g., MHC pep-
tides, CD54, CD80) to IL-2-expressing CD4?Th cells, then these
recipient T cells might also have full capacity to in turn present
these tumor epitopes to CD8?T cells and thereby initiate produc-
tive antitumor CTL responses (Fig. 1C). In this study, we exam-
ined membrane molecule transfer from APCs to CD4?T cells by
APC stimulation. We found that CD4?T cells cannot only acquire
Research Unit, Saskatchewan Cancer Agency, Departments of Oncology, Microbi-
ology, and Immunology, College of Medicine, University of Saskatchewan, Saska-
toon, Saskatchewan, Canada
Received for publication August 30, 2004. Accepted for publication March 25, 2005.
The costs of publication of this article were defrayed in part by the payment of page
charges. This article must therefore be hereby marked advertisement in accordance
with 18 U.S.C. Section 1734 solely to indicate this fact.
1This work was supported by Research Grants MOP63259 and 67230 from the Ca-
nadian Institute of Health Research (to J.X.). Y.L. was supported by Postdoctoral
Fellowship Award from the Canadian Institute of Health Research.
2Address correspondence and reprint requests to Dr. Jim Xiang, Research Unit,
Saskatchewan Cancer Agency, Departments of Oncology, Microbiology and Immu-
nology, University of Saskatchewan, 20 Campus Drive, Saskatoon, Saskatchewan
S7N 4H4, Canada. E-mail address: email@example.com
3Abbreviations used in this paper: CD40L, CD40 ligand; KO, knockout; DC, den-
dritic cell; DCOVA, OVA-pulsed DC; LB27OVAII, OVAII peptide-pulsed LB27 cells;
ECD, energy-coupled dye.
The Journal of Immunology
Copyright © 2005 by The American Association of Immunologists, Inc.0022-1767/05/$02.00
the synapse-composed MHC class II and costimulatory molecules
(CD54 and CD80), but also the bystander MHC class I-peptide
complexes from APCs and these acquired molecules on CD4?T
cells are functional. The CD4?T cells carrying acquired APC
Ag-presenting machinery can thus act as CD4?Th-APCs in stim-
ulation of CTL responses in vitro and in vivo.
Materials and Methods
Tumor cells, reagents, and animals
The highly lung metastatic B16 mouse melanoma BL6-10 and OVA-trans-
fected BL6-10 (BL6-10OVA) cell lines were generated in our own labora-
tory (22). Both cell lines form numerous lung metastasis after i.v. tumor
cell (0.5 ? 106cells/mouse) injection. The mouse B cell hybridoma cell
line LB27 expressing both H-2Kband Iab, the mouse thymoma cell line
EL4 of C57BL/6 mice, and the OVA-transfected EL4 (EG7) cell line,
which is sensitive to CTL killing, were obtained from American Type
Culture Collection. Both BL6-10 and BL6-10OVAexpress similar level of
H-2Kb, but not Iab(data not shown). Both BL6-10OVAand EG7 cells ex-
pressed OVA by flow cytometric analysis, whereas BL6-10 and EL4 cells
did not (Fig. 2). T cell hybridoma cell line RF3370 expresses TCR specific
for H-2Kb-OVA peptide complexes (23). The biotin-labeled mAbs specific
for H-2Kb(AF6-88.5), Iab(AF6-120.1), CD3 (145-2C11), CD4 (GK1.5),
CD8 (53-6.7), CD11b (MAC-1), CD11c (HL3), CD25 (7D4), CD54 (3E2),
CD69 (H1.2F3), CD80 (16-10A1), and V?2V?5?TCR (MR9-4) were
obtained from BD Pharmingen. The OVAI (SIINFEKL) and OVAII
(ISQAVHAAHAEINEAGR) peptides (24, 25) are OVA tumor peptides for
H-2Kband Iab, respectively, whereas Mut1 (FEQNTAQP) peptide is an
irrelevant 3LL lung carcinoma for H-2Kb(26). These peptides were syn-
thesized by Multiple Peptide Systems. The OVA-specific TCR transgenic
OT I and OT II mice and H-2Kb, Iab, CD4, CD8, CD54, and CD80 KO
mice on a C57BL/6 background were obtained from The Jackson Labo-
ratory. Homozygous OT II/H-2Kb?/?, OT II/Iab?/?, OT II/CD54?/?, and
OT II/CD80?/?mice were generated by backcrossing the designated gene
KO mice (H-2Kb) onto the OT II background for three generations; ho-
mozygosity was confirmed by PCR according to The Jackson Laboratory’s
protocols. All mice were maintained in the animal facility at the Saskatoon
Cancer Center and treated according to animal care committee guidelines
of the University of Saskatchewan.
Preparation of DCs
Activated, mature bone marrow-derived DCs, expressing high levels of
MHC class II, CD40, CD54, and CD80, were generated from C57BL/6
mice as described previously (27). To generate OVA-pulsed DCs
(DCOVA), DCs were pulsed overnight at 37°C with 0.1 mg/ml OVA (Sig-
ma-Aldrich, then washed extensively (26).
Preparation of OT II CD4?and OT I CD8?T cells
Naive OVA-specific CD4?T and CD8?T cells were isolated from OT II
or OT I mouse spleens, respectively, and enriched by passage through
nylon wool columns. CD4?and CD8?cells were then purified by negative
selection using anti-mouse CD8 (Ly2) or CD4 (L3T4) paramagnetic beads
(Dynal Biotech) to yield populations that were ?98% CD4?/V?2V?5?or
CD8?/V?2V?5?, respectively. To generate DCOVA-activated CD4?T
cells, CD4?T cells (2 ? 105cells/ml) from OT II mice or designated
gene-deleted OT II mice were stimulated for 3 days with irradiated (4000
rad) bone marrow-derived DCOVA(1 ? 105cells/ml) in the presence of
IL-2 (10 U/ml), IL-12 (5 ng/ml), and anti-IL-4 Ab (10 ?g/ml) (R&D Sys-
tems) (28). These in vitro DCOVA-activated CD4?T cells, also referred to
herein as CD4?Th-APCs, were then isolated by Ficoll-Paque (Sigma-
Aldrich) density gradient centrifugation or further purified using CD4 mi-
crobeads (Miltenyi Biotec) in some experiments. Con A-stimulated OT II
CD4?T (Con A-OT II) cells were similarly generated by incubating
splenocytes from OT II or OT II/KO mice with Con A (1 ?g/ml) and IL-2
(10 U/ml) for 3 days, after which the CD4?T cells were purified on density
gradients. To ascertain that no DCs were in purified Th-APCs or Con A-OT
II cells, these active T cells were further purified by using CD4 microbeads
Phenotypic characterization of DCOVA-activated CD4?T cells
For the phenotypic analyses, Th-APCs were stained with Abs specific for
H-2Kb, Iab, CD3, CD4, CD8, CD11b, CD11c, CD25, CD54, CD69, CD80,
and V?2V?5?TCR (BD Pharmingen), respectively, and analyzed by flow
cytometry. For the intracellular cytokines, cells were restimulated with
4000 rad-irradiated BL27 tumor cells pulsed with OVAII peptide for 4 h
(28), and then processed using a Cytofix/CytoPerm Plus with GolgiPlug kit
Th to CD8?CTL. A, The “passive,” three-cell interac-
tion model, in which APCs simultaneously present Ag
to the Th and the CTL, but deliver costimulatory signals
only to the helper. The CD4?Th cell in turn produces
IL-2, which is required for CTL activation. B, The dy-
namic model of sequential two-cell interactions by
APCs, in which the APC offers costimulatory signals to
the CD4?Th, which reciprocally “licenses” the APC
(left side) such that it can only then directly costimulate
the CTL (right side). C, The new dynamic model of
sequential two-cell interactions, in which APCs license
CD4?Th cells to act as APCs (Th-APCs). APCs di-
rectly transfer MHC class I-Ag complexes and costimu-
latory molecules to expanding populations of IL-2-pro-
ducing Th cells, which thereby act directly as Th1-
APCs to simulate CTL activation.
Three models for the delivery of CD4?
tometry. EG7 (thick solid lines, a) and EL4 (thick dotted lines, a) and
BL6-10OVA(thick solid lines, b) and BL6-10 (thick dotted lines, b) tumor
cells were stained with the rabbit anti-OVA Ab (Sigma-Aldrich), followed
with the FITC-conjugated goat anti-rabbit IgG Ab, and then analyzed by
flow cytometry. Tumor cells stained with normal rabbit serum were used as
control populations (thin dotted lines). One representative experiment of
two is displayed.
Flow cytometric analysis of OVA expression by flow cy-
7498A NEW DYNAMIC MODEL OF TWO-CELL INTERACTIONS BY Th-APCs
7. Bennett, S. R., F. R. Carbone, F. Karamalis, R. A. Flavell, J. F. Miller, and
W. R. Heath. 1998. Help for cytotoxic-T-cell responses is mediated by CD40
signalling. Nature 393: 478–480.
8. Schoenberger, S. P., R. E. Toes, E. I. van der Voort, R. Offringa, and C. J. Melief.
1998. T-cell help for cytotoxic T lymphocytes is mediated by CD40-CD40L
interactions. Nature 393: 480–483.
9. Yang, Y., and J. M. Wilson. 1996. CD40 ligand-dependent T cell activation:
requirement of B7-CD28 signaling through CD40. Science 273: 1862–1864.
10. Bretscher, P. A. 1999. A two-step, two-signal model for the primary activation of
precursor helper T cells. Proc. Natl. Acad. Sci. USA 96: 185–190.
11. Ingulli, E., A. Mondino, A. Khoruts, and M. K. Jenkins. 1997. In vivo detection
of dendritic cell antigen presentation to CD4?T cells. J. Exp. Med. 185:
12. Ruedl, C., P. Koebel, M. Bachmann, M. Hess, and K. Karjalainen. 2000. Ana-
tomical origin of dendritic cells determines their life span in peripheral lymph
nodes. J. Immunol. 165: 4910–4916.
13. Norbury, C. C., D. Malide, J. S. Gibbs, J. R. Bennink, and J. W. Yewdell. 2002.
Visualizing priming of virus-specific CD8?T cells by infected dendritic cells in
vivo. Nat. Immunol. 3: 265–271.
14. Tite, J. P. 1990. Evidence of a role for TNF-? in cytolysis by CD4?, class II
MHC-restricted cytotoxic T cells. Immunology 71: 208–212.
15. Rathmell, J. C., M. P. Cooke, W. Y. Ho, J. Grein, S. E. Townsend, M. M. Davis,
and C. C. Goodnow. 1995. CD95 (Fas)-dependent elimination of self-reactive B
cells upon interaction with CD4?T cells. Nature 376: 181–184.
16. Lanzavecchia, A., and F. Sallusto. 2000. Dynamics of T lymphocyte responses:
intermediates, effectors, and memory cells. Science 290: 92–97.
17. Lenschow, D. J., T. L. Walunas, and J. A. Bluestone. 1996. CD28/B7 system of
T cell costimulation. Annu. Rev. Immunol. 14: 233–258.
18. Grakoui, A., S. K. Bromley, C. Sumen, M. M. Davis, A. S. Shaw, P. M. Allen,
and M. L. Dustin. 1999. The immunological synapse: a molecular machine con-
trolling T cell activation. Science 285: 221–227.
19. Viola, A., S. Schroeder, Y. Sakakibara, and A. Lanzavecchia, 1999. T lympho-
cyte costimulation mediated by reorganization of membrane microdomains. Sci-
ence 283: 680–682.
20. Hwang, I., J. F. Huang, H. Kishimoto, A. Brunmark, P. A. Peterson,
M. R. Jackson, C. D. Surh, Z. Cai, and J. Sprent. 2000. T cells can use either T
cell receptor or CD28 receptors to absorb and internalize cell surface molecules
derived from antigen-presenting cells. J. Exp. Med. 191: 1137–1148.
21. Huang, J. F., Y. Yang, H. Sepulveda, W. Shi, I. Hwang, P. A. Peterson,
M. R. Jackson, J. Sprent, and Z. Cai. 1999. TCR-mediated internalization of
peptide-MHC complexes acquired by T cells. Science 286: 952–954.
22. Kimura, A. K., and J. H. Xiang. 1986. High levels of Met-72 antigen expression:
correlation with metastatic activity of B16 melanoma tumor cell variants. J. Natl.
Cancer Inst. 76: 1247–1254.
23. Mitchell, D., S. Nair, and E. Gilboa. 1998. Dendritic cell/macrophage precursors
capture exogenous antigen for MHC class I presentation by dendritic cells. Eur.
J. Immunol. 28: 1923–1933.
24. Li, M., G. M. Davey, R. M. Sutherland, K. Christian, A. M. Lew, C. Hirst,
F. R. Carbone, and R. H. William. 2001. Cell-associated ovalbumin is cross-
presented much more efficiently than soluble ovalbumin in vivo. J. Immunol. 166:
25. Slingluff, C. 1996. Tumor antigens and tumor vaccines: peptides as immunogens.
Semin. Surg. Oncol. 12: 446–453.
26. Zhang, W., Z. Chen, F. Li, H. Kamencic, B. Juurlink, J. R. Gordon, and J. Xiang.
2003. Tumour necrosis factor-? (TNF-?) transgene-expressing dendritic cells
(DCs) undergo augmented cellular maturation and induce more robust T-cell
activation and anti-tumour immunity than DCs generated in recombinant TNF-?.
Immunology 108: 177–188.
27. Liu, Y., H. Huang, Z. Chen, L. Zong, and J. Xiang. 2003. Dendritic cells engi-
neered to express the Flt3L stimulate type 1 immune response, and induce en-
hanced cytotoxic T and natural killer cell cytotoxcities and antitumor immunity.
J. Gene Med. 5: 668–680.
28. Nishimura, T., K. Iwakabe, M. Sekimot, Y. Ohmi, T. Yahata, M. Nakui, T. Sato,
S. Habu, H. Tashiro, M. Sato, and A. Ohta. 1999. Distinct role of antigen-specific
T helper type 1 (Th1) and Th2 cells in tumor eradication in vivo. J. Exp. Med.
29. Bourgeois, C., B. Rocha, and C. Tanchot. 2002. A role for CD40 expression on
CD8?T cells in the generation of CD8?T cell memory. Science 297:
30. Azuma, M., H. Yssel, J. H. Phillips, H. Spits, and L. L. Lanier. 1993. Functional
expression of B7/BB1 on activated T lymphocytes. J. Exp. Med. 177: 845–850.
31. Sansom, D. M., and N. D. Hall. 1993. B7/BB1, the ligand for CD28, is expressed
on repeatedly activated human T cells in vitro. Eur. J. Immunol. 23: 295–298.
32. Tatari-Calderone, Z., R. T. Semnani, T. B. Nutman, J. Schlom, and H. Sabzevari.
2002. Acquisition of CD80 by human T cells at early stages of activation: func-
tional involvement of CD80 acquisition in T cell to T cell interaction. J. Immunol.
33. Tsang, J. Y., J. G. Chai, and R. Lechler. 2003. Antigen presentation by mouse
CD4?T cells involving acquired MHC class II:peptide complexes: another
mechanism to limit clonal expansion. Blood 101: 2704–2710.
34. Hudrisier, D., and P. Bongrand. 2002. Intercellular transfer of antigen-presenting
cell determinants onto T cells: molecular mechanisms and biological significance.
FASEB J. 16: 477–486.
35. Huang, H., F. Li, J. Gordon, and J. Xiang. 2002. Synergistic enhancement of
antitumor immunity with adoptively transferred tumor-specific CD4 and CD8 T
cells and intratumoral lymphotactin transgene expression. Cancer Res. 62:
36. Li, Y., N. Wang, H. Li, K. King, R. Bassi, H. Sun, A. Santiago, A. Hooper,
P. Bohlen, and D. Hicklin. 2002. Active immunization against the vascular en-
dothelial growth factor receptor flk1 inhibits tumor angiogenesis and metastasis.
J. Exp. Med. 195: 1575–1584.
37. Curtsinger, J., C. Schmidt, A. Mondino, D. Lins, R. Kedl, M. Jenkins, and
M. Mescher. 1999. Inflammatory cytokines provide a third signal for activation
of naı ¨ve CD4?and CD8?T cells. J. Immunol. 162: 3256–3262.
38. Rosenberg, A. S., T. Mizuochi, S. O. Sharrow, A. Singer. 1987. Phenotype,
specificity, and function of T cell subsets and T cell interactions involved in skin
allograft rejection. J. Exp. Med. 165: 1296–1315.
39. Rees, M. A., A.S. Rosenberg, T. I. Munitz, and A. Singer. 1990. In vivo induction
of antigen- specific transplantation tolerance to Qa1a by exposure to alloantigen
in the absence of T-cell help. Proc. Natl. Acad. Sci. USA 87: 2765–2769.
40. Wang, J. C., and A. M. Livingstone. 2003. Cutting edge: CD4?T cell help can
be essential for primary CD8?T cell responses in vivo. J. Immunol. 171:
41. Behrens, G. M., M. Li, G. M. Davey, J. Allison, R. A. Flavell, F. R. Carbone, and
W. R Heath. 2004. Helper requirements for generation of effector CTL to islet ?
cell antigens. J. Immunol. 172: 5420–5426.
42. Janssen, E. M., E. E. Lemmens, T. Wolfe, U. Christen, M. G. von Herrath, and
S. P. Schoenberger. 2003. CD4?T cells are required for secondary expansion and
memory in CD8?T lymphocytes. Nature 421: 852–856.
43. Lyakh, L. A., G. K. Koski, H. A. Young, S. E. Spence, P. A. Cohen, and
N. R. Rice. 2002. Adenovirus type 5 vectors induce dendritic cell differentiation
in human CD14?monocytes cultured under serum-free conditions. Blood 99:
44. Mailliard, R. B., Y. I. Son, R. Redlinger, P. T. Coates, A. Giermasz, P. A. Morel.
W. J. Storkus, and P. Kalinski. 2003. Dendritic cells mediate NK cell help for Th1
and CTL responses: two-signal requirement for the induction of NK cell helper
function. J. Immunol. 171: 2366–2373.
45. Wang, B., C. C. Norbury, R. Greenwood, J. R. Bennink, J. W. Yewdell, and
J. A. Frelinger. 2001. Multiple paths for activation of naive CD8? T cells:
CD4?-independent help. J. Immunol. 167: 1283–1289.
46. Mintern, J. D., G. M. Davey, G. T. Belz, F. R. Carbone, and W. R. Heath. 2002.
Cutting edge: precursor frequency affects the helper dependence of cytotoxic T
cells. J. Immunol. 168: 977–980.
47. Hou, S., X. Y. Mo, L. Hyland, and P. C. Doherty. 1995. Host response to Sendai
virus in mice lacking class II major histocompatibility complex glycoproteins.
J. Virol. 69: 1429–1434.
48. Mauri, D., T. Wyss-Coray, H. Gallati, and W. J. Pichler. 1995. Antigen-present-
ing T cells induce the development of cytotoxic CD4?T cells. I. Involvement of
the CD80-CD28 adhesion molecules. J. Immunol. 155: 118–127.
49. Taams, L. S., W. van Eden, and M. H. Wauben. 1999. Antigen presentation by
T cells versus professional antigen-presenting cells (APC): differential conse-
quences for T cell activation and subsequent T cell-APC interactions. Eur. J. Im-
munol. 29: 1543–1550.
50. Nisini, R., A. Fattorossi, C. Ferlini, and R. D’Amelio. 1996. One cause for the
apparent inability of human T cell clones to function as professional superanti-
gen-presenting cells is autoactivation. Eur. J. Immunol. 26: 797–803.
51. Satyaraj, E., S. Rath, and V. Bal. 1994. Induction of tolerance in freshly isolated
alloreactive CD4?T cells by activated T cell stimulators. Eur. J. Immunol. 24:
52. Mannie, M. D., S. K. Rendall, P. Y. Arnold, J. P. Nardella, and G. A. White.
1996. Anergy- associated T cell antigen presentation: a mechanism of infectious
tolerance in experimental autoimmune encephalomyelitis. J. Immunol. 157:
53. Patel, D. M., P. Y. Arnold, G. A. White, J. P. Nardella, and M. D. Mannie. 1999.
Class II MHC/peptide complexes are released from APC and are acquired by T
cell responders during specific antigen recognition. J. Immunol. 163: 5201–10.
54. Ossendorp, F., E. Mengede, M. Camps, R. Filius, and C. J. Melief. 1998. Specific
T helper cell requirement for optimal induction of cytotoxic T lymphocytes
against major histocompatibility complex class II negative tumors. J. Exp. Med.
7505The Journal of Immunology