Anti-human leukocyte antigen (HLA) antibodies are the main cause of kidney and hematopoietic stem cell transplant rejection. Anti-HLA formation results from various sensitization events such as previous blood transfusion, pregnancy or organ transplant. Sensitization remains a major problem in transplantation. Although anti-HLA antibody formation in non-sensitized individuals may be associated with viral infections or vaccination, research on this topic remains limited (D'Orsogna et al., 2017). How infectious agents can trigger HLA sensitization and allograft rejection is still under investigation.

The immune response against structurally similar parts of pathogens can develop cross-reactivity with self-antigens by molecular mimicry. A short-term primer response consisting of IgM antibodies occurs on the first encounter with an antigen through infectious disease or vaccination. There is a possibility of cross-reactivity between IgM antibodies against infectious agents and HLA molecules. Indeed, IgM antibodies are the antibodies that have the most antigen binding and agglutination ability, and strongly activate complement and immune reactions (Jager et al., 2017; Stastny et al., 2009). It can also cause direct donor alloreactivity (D'Orsogna et al., 2017). Theoretically, cross-reactive IgG antibodies will be produced from cross-reactive IgM antibodies by class switching. This cross-reactive IgG antibody may mimic de novo anti-HLA IgG antibody. Thus, cross-reactive IgM antibodies may indirectly increase the existing immune response to allogeneic HLA antigens. It is therefore important to demonstrate the formation and effects of these cross-reactive IgM antibodies ex vivo and/or in vivo.

Recipient candidates are evaluated in terms of the presence of any infectious diseases as well as other systemic conditions during the pretransplantation period. It is unlikely that the recipient candidates with any acute infection receive transplantation. The use of immunosuppressive agents in post-transplant increases the rate of opportunistic infections and cancer while prolonging graft survival. Cytomegalovirus (CMV) and Epstein-Barr virus (EBV) infections are the most common opportunistic viral agents associated with post-solid organ transplant immunosuppressive treatment protocols (Burton et al., 2018; Hopwood et al., 2002). These agents' infections can lead to coinfection, rejection attack, graft loss, and in some cases, reduced survival with the immunomodulatory effects of the virus (Allen et al., 2019; Fishman, 2007; Haidar and Singh, 2017; Kotton et al., 2018; Razonable and Humar, 2019). Cross-reactivity may occur between HLA molecules and IgM antibodies of the host against these frequent pathogens after transplantation (Locke et al., 2009). In our previous study with limited samples, when we examined IgM antibodies against many viral agents (EBV, CMV, Toxoplasma, Hepatitis A Virus-HAV, Rubella, Varicella zoster virus-VZV, Brucella, Mumps, Parvovirus B19, Herpes simplex virus-HSV-1, and Hepatit B Virus-HBV) ex vivo, increased cross-reactivity was detected between EBV and CMV IgM antibodies and HLA molecules (Guleç et al., 2021). Pathogens can enhance an immune response to allogeneic HLA antigens by multiple immunological mechanisms and cause transplant failure. There are some assumptions for immunological mechanisms (Amir et al., 2010; D'Orsogna et al., 2017; D'Orsogna et al., 2012; D'Orsogna et al., 2011). Allogeneic HLA molecules may show cross-reactive activation with antibodies formed by pre-existing memory B cells against viral infection or vaccination. Additionally, virus-specific memory T cells can cross-react with HLA molecules. Specific HLA molecules' cross-reactivity with different virus-specific T cells may be mediated by the same T cell receptor. And these antibodies may increase the variety of de-novo donor-specific antibodies (DSA) or HLA antibodies (Locke et al., 2009).

In our study, we aimed to evaluate the possible cross-reactivity between HLA molecules and these IgM antibodies in patients with CMV/EBV acute infection by the complement-dependent cytotoxicity (CDC) cross-reactivity assay, which mimics the CDC cross-match assay. Although solid organ recipients were predominately studied in our study, we think that our results will be instructive for both hematopoietic stem cell and solid organ transplant recipients.