ABSTRACT We have developed a murine model to study the involvement of dendritic cells (DC) in human immunodeficiency virus (HIV) routing from an inoculation site to the lymph nodes (LN). Murine bone marrow-derived DC migrate to the draining LN within 24 h after subcutaneous injection. After incubation of these cells with heat-inactivated (Hi) HIV type 1 (HIV-1), HIV RNA sequences were detected in the draining LN only. Upon injection of DC pulsed with infectious HIV, the virus recovered in the draining LN was still able to productively infect human T cells. After a vaginal challenge with Hi HIV-1, the virus could be detected in the iliac and sacral draining LN at 24 h after injection. After an intravenous challenge, the virus could be detected in peripheral LN as soon as 30 min after injection. The specific depletion of a myeloid-related LN DC population, previously shown to take up blood macromolecules and to translocate them into the LN, prevented HIV transport to LN. Together, our data demonstrate the critical role of DC for HIV routing to LN after either a vaginal or an intravenous challenge, which does not require their infection. Therefore, despite the fact that the mouse is not infectable by HIV, this small animal model might be useful to test preventive strategies against HIV.

The migration route of dendritic cells (DC) from peripheral tissues to lymph nodes (1) and their capacity to pass HIV infection to T cells (2–4) support a critical role for DC in the early events of HIV infection. It is assumed, but not yet demonstrated, that DC are the first HIV target cells in the mucosa and that their migration to draining lymph nodes will result in the transmission of HIV to T lymphocytes. Interestingly, this can be studied in a murine model, although murine cells are not susceptible to HIV infection. Indeed, (i) murine DC pre-incubated with HIV are able to transfer it to human T lymphocytes as efficiently as human DC (4); (ii) murine DC can be manipulated in vitro, re-injected, and their in vivo migration can be analysed.

Dendritic cells (DC) knockout in mice should be a powerful mean to appreciate the role of these cells in physiological or pathological situations. In order to generate such an animal model, we used a stategy based on the DC-specific expression of a suicide gene in transgenic mice. We used the herpes simplex virus type 1-thymidine kinase (HSV1-TK) which allows conditional ablation of dividing HSV1-TK expressing cells by converting the non toxic ganciclovir (GCV) into a toxic metabolite1,2. DC expression of HSV1-TK in transgenic mice was attempted with the HIV-LTR promoter which had been previously shown to preferentially direct the expression of a CAT transgene in Langerhans cells3. We generated LTR-TK transgenic mice expressing the HSV1-TK gene under the control of the HIV-LTR promoter. We showed a low but preferential expression of the transgene in DC, leading to DC depletion in spleen and thymus following GCV administration4. This depletion was often associated with a thymic atrophy and a wasting syndrome. To rule out the possibility that a transgene expression leakiness in different tissues could be responsible for these pathological findings, we analysed the effects of GCV treatment in syngenic normal mice engrafted with transgenic bone marrow cells. In this situation, HSV1-TK expression is limited to hematopoietic cells.

Antigen presentation to T lymphocytes appears to be one of the deficient step in the induction of anti-tumoral immune responses. To overcome this deficit, it should be possible to use the professional antigen presenting dendritic cells. The principle of this strategy would be to purify dendritic cells, to prime them ex vivo with tumoral antigen, and to re-inject them to patient. The purification of dendritic cells can be achieved from the spleen, bone marrow, and peripheral or cord blood. Their sensitization to tumoral antigen could be obtained using various antigeneic preparation such as crude tumoral extract, or purified antigen, that will lead to an MHC class II restricted antigenic presentation to CD4+ T cells. Gene transfer can be used in the case of a cloned antigen and would lead to the restricted MHC class I priming of CD8+ T cells. The mode of administration, the nature of the dendritic cells used, the number of sensitized cells to inject, might depend on the nature and the location of the tumour. In vitro, it has been shown that dendritic cells sensitized with tumoral antigen are capable of triggering proliferative immune responses as well as cytotoxic T cells. In vivo, injection of dendritic cells primed with tumour cell lysate leads to protection of mice against a tumour challenge. Finally, gene transfer to dendritic cells is shown hereby to be possible, although the efficacy of transduction is still very low, and must be improved. Altogether, it should soon be feasible to use ex vivo primed dendritic cells for triggering otherwise inefficient immune responses in pathologies such as cancer or HIV infection.

Antigen presentation to T lymphocytes appears to be one of the deficient step in the induction of anti-tumoral immune responses. To overcome this deficit, it should be possible to use the professional antigen presenting dendritic cells.;The principle of this strategy would be to purify dendritic cells, to prime them ex vivo with tumoral antigen, and to re-inject them to patient. The purification of dendritic cells can be achieved from the spleen, bone marrow, and peripheral or cord blood. Their sensitization to tumoral antigen could be obtained using various antigenic preparation such as crude tumoral extract, or purified antigen, that will lead to an MHC class II restricted antigenic presentation to CD4+ T cells. Gene transfer can be used in the case of a cloned antigen and would lead to the restricted MHC class I priming of CD8+ T cells. The mode of administration, the nature of the dendritic cells used, the number of sensitized cells to inject, might depend on the nature and the location of the tumour. In vitro, it has been shown that dendritic cells sensitized with tumoral antigen are capable of triggering proliferative immune responses as well as cytotoxic T cells. In vivo, injection of dendritic cells primed with tumour cell lysate leads to protection of mice against a tumour challenge. Finally, gene transfer to dendritic cells is shown hereby to be possible, although the efficacy of transduction is still very low, and must be improved. Altogether, it should soon be feasible to use ex vivo primed dendritic cells for triggering otherwise inefficient immune responses in pathologies such as cancer or HIV infection.