Soluble human major histocompatibility class I antigens: new immunomodulatory functions for old molecules

short-legendFrancesco Indiveri

The human major histocompatibility complex encodes two sets of Class I molecules, which have been termed Class Ia (or classical) and Class Ib (or nonclassical) molecules. The Class Ia molecules include the gene products of human leukocyte antigen (HLA)-A, -B and -C loci, and are characterized by broad tissue expression and by a high degree of polymorphism. The Class Ib molecules include the gene products of HLA-E, -F and -G loci and are characterized by a restricted tissue distribution and limited polymorphism.

Besides being expressed on nucleated cells, Class Ia and Ib HLA molecules are present in serum in soluble form (sHLA-I) [1,2]. The serum level of sHLA-I molecules is significantly increased in a variety of physiologic and pathologic conditions such as pregnancy, acute rejection episodes following organ allografts, acute graft-versus-host disease (GVHD) following bone marrow transplantation, autoimmune diseases, viral infections, malignant melanoma and multiple myeloma [3–8]. Due to the significant association with clinical parameters, the level of sHLA-I antigens has been suggested to represent a useful marker to predict the evolution of viral infections and to monitor the clinical course of allografts [9]. Moreover, elevated levels of functional sHLA-I molecules have been detected in blood components and might play a role in the immunomodulatory effect of autologous and allogeneic transfusions [10–13].

Several lines of evidence suggest that sHLA-I molecules are immunologically functional and may play an immunoregulatory role. In fact, they have been shown to elicit antibodies in both allogeneic and xenogeneic combinations, to inhibit the activity of alloreactive cytotoxic T lymphocytes (CTLs) [14–16], and to induce apoptosis in alloreactive and virus-specific CTLs, in activated autologous and allogeneic CD8⁺ T cells and in CD8⁺ natural killer (NK) cells [17–24].

There is general agreement about the mechanism underlying the inhibition of CTL activity by sHLA-I antigens. This inhibition appears to be mediated by the interactions of sHLA-I antigens α1 and -2 domains with T-cell receptors (TCRs) [14–16]. In contrast, there is conflicting information concerning the mechanism underlying induction of apoptosis of activated T cells by sHLA-I antigens. Several authors reported that sHLA-I molecules induced apoptosis of alloreactive CD8⁺ CTLs through interaction with their TCR [17]. However, the author’s data and those from other groups indicate that classical and nonclassical sHLA-I molecules trigger Fas/Fas-ligand (FasL) mediated apoptosis of phytohemoagglutinin (PHA)-activated and virus-specific CD8⁺ T lymphocytes as well as of CD8⁺ NK cells by interacting with the CD8 coreceptor [18–24].

Recently, the authors performed a series of experiments to clarify the intracellular mechanism(s) leading to FasL upregulation and secretion following CD8 ligation by sHLA-I molecules [25]. Results showed that sHLA-I/CD8 interaction induced the recruitment of src-like p56 ^lck and syk-like ZAP-70 protein tyrosine kinases, whereas the binding of sHLA-I to the CD3/TCR complex recruited p59 ^fyn PTK. Then, the engagement of CD8 by sHLA-I led to the activation of the Ca²⁺ calmodulin kinase II pathway which was eventually responsible for the NF-AT nuclear translocation. In contrast, sHLA-I/CD8 interaction, as opposed to signalling through the CD3/TCR complex, did not induce nuclear translocation of AP-1 protein complex. In addition, the authors found that the ligation of sHLA-I to CD8-recruited protein kinase C led to NF-kB activation. Both nuclear factor (NF) of activated T cells and NF-kB were responsible for the induction of FasL messenger RNA and consequently, CTL apoptosis. Moreover, FasL upregulation and CTL apoptotic death were downregulated by pharmacologically specific inhibitors of Ca²⁺/calmodulin/calcineurin and Ca²⁺-independent PKC signalling pathways.

Apoptosis induced by Fas/FasL interactions plays a crucial role in the establishment of antigen-specific T-cell tolerance, both during the intrathymic negative selection and adult life [26–30]. The amount of sHLA-I molecules that induces apoptosis in CD8⁺ cells is analogous to the level found in plasma of patients with an activation of their immune system. Therefore, sHLA-I antigens secreted during immune-system activation may bind to CD8 molecules on activated CD8⁺ T lymphocytes and NK cells and induce soluble FasL secretion. Soluble FasL may then act in an autocrine and/or paracrine fashion, triggering apoptosis in activated CD8⁺ CD95⁺ cells. If so, serum sHLA-I molecules may represent an important efferent arm of the network to control the expansion of CD8⁺ cells and to downregulate immune responses and could be proposed as a potential immunosuppressive tool in the field of transplantation and autoimmunity.