CD8 T cells from major histocompatibility complex class II-deficient mice respond vigorously to class II molecules in a primary mixed lymphocyte reaction

Mature CD4⁺ and CD8⁺ T cells are restricted by major histocompatibility complex (MHC) class II and class I molecules, respectively. In a primary mixed lymphocyte reaction (MLR), CD8⁺ T cells from C57BL/6 (B6) mice can respond to allo-class I molecules, but not allo-class II molecules. However, a significant fraction of CD8⁺ T cells from C57BL/6 class II-deficient (B6Aα^?) mice violate this rule by responding vigorously in a MLR to class II molecules. The frequency of responding cells is ～ 50 % of that of B6 CD8⁺ T cells responding to B6bm1 allo-class I molecules. This response requires neither appropriate co-receptor, i.e. CD4, nor exogenous lymphokines, indicating that interactions between the T cell receptors (TCR) and class II molecules are remarkably efficient. Since these CD8⁺ T cells are positively selected by class I molecules in the thymus of class II-deficient mice, these CD8⁺ T cells should interact with both classes of MHC molecules. The absence of thymic negative selection by class II molecules may result in the production of these CD8⁺ T cells. The data imply that a substantial fraction of CD4⁺CD8⁺ double-positive thymocytes in wild-type mice interacts with both classes of MHC molecules prior to thymic selection.     

Activation of virus-specific major histocompatibility complex class II-restricted CD8+ cytotoxic T cells in CD4-deficient mice

Acute enteritic or respiratory disease is a consequence of coronavirus infection in man and rodents. Mouse hepatitis virus, stain A59 (MHV-A59) causes acute hepatitis in mice and rats and induces a response of major histocompatibility complex (MHC) class II-restricted CD4⁺ cytotoxic T cells, protecting mice against acute infection. In the present study we show that MHV-A59 infection of mice that lack a functional CD4 gene activates effector cells of the CD8⁺ phenotype. These cytotoxic T cells lyse virus-infected target cells in a MHC class II-restricted fashion. The results indicate that CD8⁺ T cells have the potential to utilize MHC class II as restriction element, illustrating that the immune system can effectively deal with evading microorganisms, such as viruses which down-regulate MHC class I.     

Major histocompatibility complex class I molecules are required for the development of insulitis in non-obese diabetic mice

An early step in the development of autoimmune diabetes is lymphocyte infiltration into the islets of Langerhans of the pancreas, or insulitis. The infiltrate contains both CD4⁺ and CD8⁺ T cells and both are required for progression to diabetes in non-obese diabetic (NOD) mice. It has been thought that the CD4⁺ lymphocytes are the initiators of the disease, the islet invaders, while CD8⁺ cells are the effectors, the islet destroyers. We question this interpretation because NOD mice lacking MHC class I molecules, hence CD8⁺ T cells, do not display even insulitis when expected.     

Lack of oral tolerance but oral priming for contact sensitivity to dinitrofluorobenzene in major histocompatibility complex class II-deficient mice and in CD4+ T cell-depleted mice

Oral tolerance is defined by immune unresponsiveness after oral administration of soluble antigens and by antigen-specific inhibition of peripheral immune responses induced by prior antigen feeding. The aim of this study was to investigate the implication of the major histocompatibility complex (MHC) class II presentation pathway to CD4⁺ T cells in oral tolerance of contact sensitivity (CS) to the hapten dinitrofluorobenzene (DNFB). We used MHC class II knockout (ABº/º) and invariant chain knockout (Iiº/º) mice, which have, respectively, a total or partial defect in class II-restricted activation of CD4⁺ T cells, as well as normal C57BL/6 mice depleted of CD4⁺ T cells by injection of a specific antibody. Intragastric administration of DNFB prior to skin sensitization induced specific inhibition of contact sensitivity to DNFB in Aβ⁺/º and Ii⁺/º heterozygotes comparable to that observed in C57BL/6 mice. In contrast, no oral tolerance was observed in either MHC class II-deficient Aβº/º and Iiº/º homozygote mutants or in syngeneic anti-CD4-depleted C57BL/6 mice. Moreover, a single oral administration of DNFB, without skin sensitization, could prime Aβº/º, Iiº/º as well as anti-CD4-depleted C57BL/6 mice for DNFB-specific CS. These findings demonstrate that the class II/CD4 pathway is involved in oral tolerance manifested both as the inhibition of CS by hapten feeding prior to skin sensitization, and as immune unresponsiveness of normal mice to oral administration of hapten. Furthermore, our data provide evidence that a single oral feeding with DNFB is able to prime mice for hapten-specific CS, provided that the class II/CD4 pathway is bypassed.     

Naive alloreactive CD8 T cells are activated by purified major histocompatibility complex class I and antigenic peptide

In this report, we demonstrate stimulation of T cell receptor (TCR) transgenic CD8 T cells by isolated major histocompatibility complex (MHC) class I H-2L^d complexes and antigenic peptide. This is the first demonstration of CD8 T cells activated by MHC and antigenic peptide in the absence of antigen priming. Furthermore, isolated MHC and a potent peptide antigen can stimulate phenotypically naive CD44^? T cells to become CTL effectors and to produce interleukin-2 in nanogram per milliliter amounts. These results demonstrate that particular TCR antigen pairs may overcome the need for specialized antigen-presenting cells and have implications for mechanisms of autoimmunity and tolerance induction.     

Reduced development of CD4−8−B220+ T cells but normal autoantibody production in lpr/lpr mice lacking major histocompatibility complex class I molecules

The lpr gene has recently been shown to encode a functional mutation in the Fas receptor, a molecule involved in transducing apoptotic signals. Mice homozygous for the lpr gene develop an autoimmune syndrome accompanied by massive accumulation of double-negative (DN) CD4^?8^?B220⁺ T cell receptor-α/β⁺ cells. In order to investigate the origin of these DN T cells, we derived lpr/lpr mice lacking major histocompatibility complex (MHC) class I molecules by intercrossing them with β2-microglobulin (β2m)-deficient mice. Interestingly, these lprβ2m–/– mice develop 13-fold fewer DN T cells in lymph nodes as compared to lpr/lpr wild-type (lprWT) mice. Analysis of anti-DNA antibodies and rheumatoid factor in serum demonstrates that lprβ2m–/– mice produce comparable levels of autoantibodies to lprWT mice. Collectively our data indicate that MHC class I molecules control the development of DN T cells but not autoantibody production in Ipr/lpr mice and support the hypothesis that the majority of DN T cells may be derived from cells of the CD8 lineage.     

Contribution of α/β and γ/δ T lymphocytes to immunity against Mycobacterium bovis Bacillus Calmette Guérin: studies with T cell receptor-deficient mutant mice

Mutant mice with defined T cell deficiencies were infected with Mycobacterium bovis bacillus Calmette Guérin (BCG) and the relative contribution of α/β T cells and γ/δ T cells to the host immune response was assessed. Recombinase activating gene (RAG-1)^?/? mutants as well as T cell receptor (TcR) β^?/?, but not TcR-δ^?/?, mutants succumbed to M. bovis BCG infection and failed to develop granulomatous lesions. Antigen-induced IFN-γ production by spleen cells in vitro was abrogated in RAG-1^?/? mutants and markedly diminished in TcR-β^?/? and TcR-δ^?/? mice. Reconstitution experiments suggest that both α/β and γ/δ T cells are essential for antigen-specific IFN-γ secretion. Our data formally prove the crucial role of α/β T cells and reveal accessory functions of γ/δ T cells in optimum immunity against M. bovis BCG.     

Lymphocyte-activation gene 3/major histocompatibility complex class II interaction modulates the antigenic response of CD4+ T lymphocytes

The activation requirements for antigen-dependent proliferation of CD4⁺ T cells are well documented, while the events leading to the inactivation phase are poorly understood. Here, we tested the hypothesis that the lymphocyte-activation gene 3 (LAG-3), a second major histocompatibility complex (MHC) class II ligand, plays a regulatory role in CD4⁺ T lymphocyte activation. CD4⁺ class II-restricted T cell clones were stimulated by their relevant antigen (hemagglutinin peptide or diphteria toxoid) and antigen-presenting cells with or without anti-LAG-3 monoclonal antibody (mAb). Kinetic studies were performed to monitor different activation parameters, including the measurement of thymidine incorporation, expression of activation antigens and cytokine secretion. Results showed that the time course from the initial time points up to the peak time point was not modified in the presence of anti-LAG-3 mAb. However, addition of these antibodies, either as whole IgG or as Fab fragments, led to increased thymidine incorporation values for late time points and, hence, to a shift in the decreasing proliferation curve. We also showed that expression of activation antigens, such as CD25, was higher in the presence of anti-LAG-3 mAb, and that cytokine concentrations, i.e. of interferon-γ or interleukin-4, were higher in the corresponding culture supernatants. In addition, we tested whether the effects of anti-LAG-3 mAb were limited to antigen-dependent. MHC class II-restricted responses. The proliferative responses of CD4⁺ T cell clones following stimulation with either interleukin-2, mitogens, a combination of anti-CD2 mAb, immobilized anti-CD3 or anti-T cell receptor mAb were not altered by anti-LAG-3 mAb. The allogeneic proliferative response of a CD8⁺ T cell clone was also not affected. Overall, the present analysis reveals a modulating effect of anti-LAG-3 mAb, mediated specifically on antigen-dependent, MHC class II-restricted responses of CD4⁺ T cell lines. These results support the view that LAG-3/MHC class II interaction down-regulates antigen-dependent stimulation of CD4⁺ T lymphocytes.     

T cell major histocompatibility complex class II molecules down-regulate CD4+ T cell clone responses following LAG-3 binding

T cell response to its antigen requires recognition by the T cell receptor together with a co-receptor molecule, either CD4 or CD8. Additional molecules have been identified that are capable of delivering the co-stimulatory signals provided by APC. Following T cell priming, a number of T cell activation antigens are expressed that may play a role in the inactivation phase of the T cell response. The lymphocyte activation gene (LAG)-3 protein and its counter-receptors, the major histocompatibility complex (MHC) class II molecules, are such activation antigens whose interaction may result in the down-regulation of the ongoing immune response. To investigate the role of LAG-3/class II molecule interaction, we produced a soluble form of LAG-3 by fusing the extracellular Ig domains of this membrane protein to the constant region of human IgG1 (LAG-31g). Here, we show a direct and specific binding of LAG-3Ig to class II molecules on the cell surface. In addition, we show that LAG-3/class II molecule interaction leads to the down-regulation of CD4⁺ Ag-specific T cell clone proliferation and cytokine secretion. This inhibitory effect is observed at the level of the effector cells and not the APC and is also found with anti-CD3 mAb, PHA + PMA or low-dose IL-2 driven stimulation in the absence of APC. These functional studies indicate that T cell MHC class II molecules down-regulate T cell proliferation following LAG-3 binding and suggest a role for LAG-3 in the control of the CD4⁺ T cell response.     

Presentation of Listeria monocytogenes antigens by major histocompatibility complex class I molecules to CD8 cytotoxic T lymphocytes independent of listeriolysin secretion and virulence

Virulence and intracellular persistence of Listeria monocytogenes markedly depend on secretion of listeriolysin (Hly), which promotes invasion of the pathogen from the endosome into the cytosol. Recent studies have provided compelling evidence that Hly also facilitates recognition of listerial antigens, in association with major histocompatibility complex (MHC) class I molecules, by CD8 T lymphocytes. Data presented here confirm that the Hly-deficient strains, the prfA^? mutant L. monocytogenes SLCC53 and the transposon mutants L. monocytogenes M3 and M20 are avirulent for mice, and unable to replicate inside bone marrow-derived macrophages (BMMΦ). Furthermore, BMMΦ infected with M3, M20 or SLCC53 were as efficiently lysed as BMMΦ infected with the Hly-positive wild-type strain EGD by MHC class I-dependent CD8 cytotoxic T lymphocytes. Using the highly sensitive polymerase chain reaction method, hly mRNA was detectable in BMMΦ infected with L. monocytogenes EGD or SLCC53, but totally absent in M3-infected BMMΦ. In the case of M20, an excision of the transposon occurred, but the excision was not precise and the hly gene was approximately 400 base pairs shorter. These findings argue against a unique role for Hly in MHC class I presentation of listerial antigens, although Hly appears central to virulence and intracellular replication. Thus, virulence of L. monocytogenes is dissociable from MHC class I presentation of listerial antigens.     

Major histocompatibility complex class I-restricted CD8+ T cells and class II-restricted CD4+ T cells,respectively, mediate and regulate contact sensitivity to dinitrofluorobenzene

Contact sensitivity (CS) is a form of delayed-type hypersensitivity to haptens applied epicutaneously and is thought to be mediated, like classical delayed-type hypersensitivity responses, by CD4⁺ T helper-1 cells. The aim of this study was to identify the effector T cells involved in CS. We studied CS to the strongly sensitizing hapten dinitrofluorobenzene (DNFB) in mice rendered deficient by homologous recombination in either major histocompatibility complex (MHC) class I, MHC class II, or both, and which exhibited deficiencies in, respectively, CD8⁺, CD4⁺, or both, T cells. MHC class I single-deficient and MHC class I/class II double-deficient mice, both of which have a drastic reduction in the number of CD8⁺ T cells, were unable to mount a CS response to DNFB. In contrast, both MHC class II-deficient mice and normal mice treated with an anti-CD4 monoclonal antibody (mAb) developed exaggerated and persistent responses relative to heterozygous control littermates. Furthermore, anti-CD8 mAb depletion of class II-deficient mice totally abolished their ability to mount an inflammatory response to DNFB. Removal of residual CD4⁺ T cells in class II-deficient mice by anti-CD4 mAb treatment did not diminish the intensity of CS. These data clearly demonstrate that class I-restricted CD8⁺ T cells are sufficient for the induction of CS to DNFB, and further support the idea that MHC class II-restricted CD4⁺ T cells down-regulate this inflammatory response.     

Trypanosoma cruzi infection does not impair major histocompatibility complex class I presentation of antigen to cytotoxic T lymphocytes

Trypanosoma cruzi (T. cruzi), the etiological agent of Chagas' disease, lives free within the cytoplasm of infected host cells. This intracellular niche suggests that parasite antigens may be processed and presented on major histocompatibility complex (MHC) class I molecules for recognition by CD8⁺ T cells. However, the parasite persists indefinitely in the mammalian host, indicating its success at evading immune clearance. It has been shown that T. cruzi interferes with processing and presentation of antigenic peptides in the MHC class II pathway. This investigation sought to determine whether interference in MHC class I processing and presentation occurs with T. cruzi infection. Surface expression of MHC class I molecules was found to be unaffected or up-regulated by T. cruzi infection in vitro. A model system employing a β-galactosidase (β-gal)-specific murine cytotoxic T lymphocyte (CTL) line (0805B) showed: (i) in vitro infection of mouse peritoneal macrophages or J774 cells with T. cruzi did not inhibit MHC class I presentation of exogenous peptide (a nine-amino acid epitope of β-gal) to the CTL line, (ii) in vitro infection of a β-gal-expressing 3T3 cell line (LZEJ) with T. cruzi did not inhibit MHC class I presentation of the endogenous protein to the CTL line and (iii) mouse renal adenocarcinoma cells infected with T. cruzi and subsequently infected with adenovirus expressing β-gal were able to present antigen to the β-gal-specific CTL line. These findings indicate that the failure of the immune response to clear T. cruzi does not result from global interference by the parasite with MHC class I processing and presentation. Parasites engineered to express β-gal were unable to sensitize infected antigen-presenting cells in vitro to lysis by the CTL 0805B line. This was probably due to the intracellular localization of the β-gal within the parasite and its inaccessibility to the host cell cytoplasm.     

Expression of class II,but not class I,major histocompatibility complex molecules is required for granuloma formation in infection with Schistosoma mansoni

Previous studies have suggested that granulomatous inflammation in schistosomiasis is mediated by CD4⁺ T helper lymphocytes sensitized to parasite egg antigens. However, CD8⁺ T cells have also frequently been associated with the immune response to schistosome eggs. To examine more precisely the role of CD4⁺ and CD8⁺ T cells in the pathology of the schistosomal infection, we used mice with targeted mutations in major histocompatibility complex (MHC) class II or class I molecules. These mutations lead, respectively, to the virtual absence of CD4⁺ and CD8⁺ T cells. The results clearly show that schistosome-infected MHC class II mutant mice failed to form granulomas around parasite eggs. In contrast, infected MHC class I mutant mice displayed characteristic granulomatous lesions that were comparable to those in wild-type control mice. Moreover, lymphoid cells from MHC class II mutant mice were unable to react to egg antigens with either proliferative or cytokine [interferon-gamma, interleukin (IL)-4, IL-10] responses; nor were they able to present egg antigens to specifically sensitized CD4⁺ T helper cells from infected syngeneic control mice. By comparison, cells from MHC class I mutant mice exercised all these functions in a manner comparable with those from wild-type controls. These observations clearly demonstrate that schistosomal egg granulomas are mediated by MHC class II-restricted CD4⁺ T helper cells. They also suggest that CD8⁺ T cells do not become sensitized to egg antigens and play little role, if any, in the pathogenesis of schistosomiasis.     

Identification of conserved T cell receptor CDR3 residues contacting known exposed peptide side chains from a major histocompatibility complex class I-bound determinant

We have analyzed the T cell receptor (TCR) repertoire found in the major histocompatibility complex class I-restricted cytotoxic T lymphocyte (CTL) response to the protein ovalbumin (OVA). Despite skewing towards the expression of Vβ5.2⁺ TCR by OVA-specific CTL from C57BL/6 mice, we found a relatively high degree of diversity in V(D)J usage in both TCR α- and β-chains. Closer examination showed that the majority of these sequences encoded negatively and positively charged residues at their respective TCR α- and β-chain VJ or VDJ junctions. These junctions form the third complementarity-determining regions (CDR3) of the TCR polypeptides involved in the direct interaction with the class I-bound peptide. Crystallographic analyses of K^b-peptide complexes predict that the major determinant from OVA, peptide OVA_257–264 (SIINFEKL), contains two exposed charged side chains which can contact the TCR. These are the negatively charged glutamic acid at determinant position 6 (P6) and the positively charged lysine at P7. To examine whether the TCR α-chain makes contact with P7 lysine, we established a single chain TCR transgenic C57BL/6 mouse line where all T cells express a TCR β-chain derived from the Vβ5.2⁺ clone B3. OVA-specific T cells derived from in vivo primed transgenic mice preferentially expressed TCR α-chains that also contained negatively charged junctional residues despite some further variation in Vα and Jα sequences. Stimulation of naive TCR β-chain transgenic T cells with a P7 substitution peptide analogue induced a T cell response that was no longer cross-reactive with the wild-type OVA_257–264 determinant, sugesting that the TCR α-chain from the T cell clone B3 can determine the specificity for this residue. Consequently, these results reveal the existence of conserved residues in the CDR3 of TCR α- and β-chains specific for OVA_257–264 and identify their possible orientation over the peptide-class I complex.     

Cross-reactivity among evolutionarily distant major histocompatibility complex class I molecules (HLA-B27 and H-2Kk) revealed by xenoreactive T lymphocytes

A set of mouse HLA-B27-reactive cytotoxic T lymphocyte clones were found to recognize the HLA-B27 molecule in an H-2-unrestricted manner, i.e. independently of any mouse major histocompatibility complex (MHC) molecule. The reactivity patterns of these clones on HLA-B27 variants (positive only on HLA-B*2702 and HLA-B*2701) allowed the identification of residues N77 and A81 of the HLA-B27 molecule as important for their reactivity. The location of these residues in the peptide-binding groove (specificity pocket F) suggested that the reactivity of the clones is dependent on HLA-B27-bound peptide(s). However, several other class I molecules sharing these residues (N77 and A81) were not recognized, indicating that other residues might also be involved. One of the clones was found to display an interesting cross-reactivity with allogeneic H-2K^k molecules, sharing N77 and A81 with HLA-B*2702. Sequence comparison suggested the involvement of residue H9, located in specificity pocket B of the peptide-binding groove, and revealed some similarity of pockets B in HLA-B27 and H-2K^k. The structural basis of such T cell-mediated MHC cross-reactions across species barriers is discussed.     

A comparison of the contribution of the major histocompatibility complex (MHC) and Y chromosomes to the discriminability of individual urine odors of mice by Long-Evans rats

A go/no-go operant task was used to assess the ability of male Long-Evans rats to discriminate between the urine odors from pairs of intact MHC congenic mice (C57BL/6-H-2K^b/J and C57BL/6-H-2K^bm1/ByJ), intact Y congenic mice (DBA1 and DBA1.C57BL10-Y), and castrated Y congenic mice of these two strains. The MHC congenic strains differ in alleles of the H-2 K locus, while the Y congenic strains differ in the nonrecombining part of the Y chromosome. Analysis of the number of correct responses to a criterion of 85% correct on each block of 20 trials revealed that the ability of the subjects to discriminate between urine odors did not differ whether samples were from pairs of intact MHC congenic mice, intact Y congenic mice, or castrated Y congenic mice. These findings are consistent with the hypothesis that individually unique urine odors may be influenced both by genes in the nonrecombining part of the Y chromosome and by genes in the major histocompatibility complex of chromosome 17. These odors are not androgen dependent. Such urinary chemical signals may be involved in pregnancy block (the Bruce effect), aggression, and other mouse social behaviors.     

A role for the Hsp90 molecular chaperone family in antigen presentation to T lymphocytes via major histocompatibility complex class II molecules

The heat shock protein (HSP) Hsp90 is known to chaperone cytosolic peptides for MHC class I (MHCI)-restricted antigen presentation to T lymphocytes. We now demonstrate a role for Hsp90 activity in presentation of antigens on MHCII. Treatment of mouse antigen-presenting cells (APC) with the pharmacological Hsp90 inhibitor, geldanamycin, inhibited MHCII-mediated presentation of endocytosed and cytosolic proteins as well as synthetic peptides to specific T cells. Ectopic expression of human Hsp90 in APC enhanced MHCII-mediated antigen presentation. Further, pharmacological Hsp90 inhibition reduced, while retroviral Hsp90 overexpression enhanced, the levels of stable compact MHCII heterodimers correlating with the antigen presentation phenotype. Pharmacological inhibition of Hsp90 activity in IFN-gamma-treated APC resulted in severe abrogation of MHCII-restricted presentation of cytosolic antigen, but only partially inhibited exogenous antigen presentation. Our data suggest a major role for Hsp90 activity in MHCII-mediated antigen presentation pathways, and implicate IFN-gamma-inducible Hsp90-independent mechanisms.     

Carboxy-terminal residues of major histocompatibility complex class II-associated peptides control the presentation of the bacterial superantigen toxic shock syndrome toxin-1 to T cells

Previous studies have shown that the presentation of some bacterial superantigens by major histocompatibility complex (MHC) class II molecules is strongly influenced by class II-associated peptides. For example, presentation of the toxic shock syndrome toxin-1 (TSST-1) superantigen by antigen-processing-defective T2-I-A^b cells (which expresses I-A^b that is either empty or associated with invariant chain-derived peptides) can be strongly enhanced by some, but not other, I-A^b-binding peptides. Here we investigate the contribution of I-A^b-associated peptides in the presentation of TSST-1 to T cells. The data show that overlapping peptides expressing the same core I-A^b-restricted epitope, but with various N and C termini, can differ profoundly in their ability to promote TSST-1 presentation to T cells. Analysis of altered and truncated peptides indicates that residues at the C-terminal end of the peptide have a dramatic effect on TSST-1 presentation. This effect does not involve a cognate interaction between the peptide and the TSST-1 molecule, but appears to depend on the length of the C-terminal region. These data are consistent with crystallographic studies suggesting that TSST-1 may interact with the C-terminal residues of MHC class II-associated peptides. We also examined the capacity of naturally processed peptides to promote TSST-1 binding using a superantigen blocking assay. The data demonstrated that a naturally processed epitope is dominated by peptides that do not promote strong TSST-1 binding to I-A^b. Taken together, these data suggest that TSST-1 binding to MHC class II molecules is controlled by the C-terminal residues of the associated peptide, and that many naturally processed peptide/class II complexes do not present TSST-1 to T cells. Thus, the peptide dependence of TSST-1 binding to class II molecules may significantly reduce the capacity of TSST-1 to stimulate T cells.     

Crosstalk between human DC subsets promotes antibacterial activity and CD8+ T‐cell stimulation in response to bacille Calmette‐Guérin

To date, little is known about the unique contributions of specialized human DC subsets to protection against tuberculosis (TB). Here, we focus on the role of human plasmacytoid (p)DCs and myeloid (m)DCs in the immune response to the TB vaccine bacille Calmette‐Guérin (BCG). Ex vivo DC subsets from human peripheral blood were purified and infected with BCG expressing GFP to distinguish between infected and noninfected cells. BDCA‐1⁺ myeloid DCs were more susceptible than BDCA‐3⁺ mDCs to BCG infection. Plasmacytoid DCs have poor phagocytic activity but are equipped with endocytic receptors and can be activated by bystander stimulation. Consequently, the mutual interaction of the two DC subsets in response to BCG was analyzed. We found that pDCs were activated by BCG‐infected BDCA‐1⁺ mDCs to upregulate maturation markers and to produce granzyme B, but not IFN‐α. Reciprocally, the presence of activated pDCs enhanced mycobacterial growth control by infected mDCs and increased IL‐1β availability. The synergy between the two DC subsets promoted BCG‐specific CD8⁺ T‐cell stimulation and the role of BCG‐infected BDCA‐1⁺ mDCs could not be efficiently replaced by infected BDCA‐3⁺ mDCs in the crosstalk with pDCs. We conclude that mDC–pDC crosstalk should be exploited for rational design of next‐generation TB vaccines.