Question

Why in organ transplantation the dendritic cells can activate the T cells but not in hematopoietic stem cell transplantation? In the both following cases let's consider that the donor has different HLA type from the recipient. When someones let's say kidney is transplanted into someone that has different HLA alleles, the dendritic cell from the graft goes to the lymph nodes and the T cells recognise that the dendritic cells have a foreign HLA type, hence the T cells are activated and go to kill the cells of the kidney --> rejection This showed that even different HLA molecules are able to activate the T cells. Then how come in hematopoietic stem cell transplantation, the dendritic cells (during an infection) of the donor which have the donors' HLA type, are not able to activate the T cells of the recipient that has its own HLA type?

Answer 1

In organ transplants, donor dendritic cells can activate recipient T cells, leading to rejection due to the different HLA types. In hematopoietic stem cell transplants, the donor's immune cells eventually see the recipient's body as 'self', which can lead to graft-versus-host disease instead.

Step-by-step explanation:

The difference in the immune response to organ transplantation versus hematopoietic stem cell transplantation (HSCT) can be attributed to the types of cells involved and their roles in the immune system. In organ transplantation, dendritic cells from the donor organ can act as antigen-presenting cells (APCs), recognizing the recipient's tissue as foreign due to different human leukocyte antigens (HLA types). These dendritic cells present antigens to T cells in the recipient, leading to an immune response and potential rejection of the transplanted organ.

However, in HSCT, the situation is somewhat reversed. The transplanted hematopoietic stem cells from the donor will eventually give rise to new immune cells, including dendritic cells, which will have the donor's HLA type. A successful HSCT requires the elimination or suppression of the recipient's immune system to allow the donor cells to engraft. This is an intentional effect that prevents the recipient's immune system from destroying the transplanted stem cells. As the new donor-derived immune system develops, it will then see the recipient's body as 'self' and not mount an immune response against it, although this can lead to graft-versus-host disease (GVHD) if donor T cells recognize recipient tissues as foreign. GVHD is a specific complication of HSCT, not seen in solid organ transplants.