In this report, we describe the unique features of HHV-6-associated lymphadenitis in two adults presenting with an acute febrile illness. Lymph node biopsies showed marked paracortical expansion due to a proliferation of large atypical cells resembling immunoblasts. Numerous intranuclear and cytoplasmic inclusions were seen in expanded paracortical areas. Immunohistochemical studies showed that the cells containing the inclusions were CD3/CD4-positive T lymphocytes, and electron microscopic analysis demonstrated numerous viral particles in the cytoplasm and nuclei of the infected cells, consistent with the herpesvirus family. Immunohistochemical staining for HHV-6 using an antibody against the envelope glycoprotein gp60/110 kDa was positive, while immunohistochemical stains for HSV-1, CMV, EBV, and HHV-8 were negative. PCR and sequence analysis confirmed HHV-6 with a type B genotype.
The florid and monomorphic expansion of the paracortex has not been described in the previous report of HHV6-associated lymphoproliferative disorders.10
Initially, the atypical cytological features of the virally infected cells raised the possibility of involvement by non-Hodgkin’s lymphoma with immunoblastic features. However, the lack of an aberrant phenotype and absence of evidence of a clonal T-cell process did not support this diagnosis. We also considered CMV lymphadenitis in the differential diagnosis. CMV infection is usually associated with viral inclusions in endothelial cells. However, in rare instances CMV infection of T cells has been described.19
Such cells may contain characteristic owl’s eye inclusion bodies in expanded paracortical areas.
Since most people become infected with HHV-6 during childhood, reports of symptomatic HHV-6 infections in adults are relatively uncommon. Mononucleosis-like illness due to HHV-6 with fatigue, cervical lymphadenopathy, sore throat, and elevated liver enzymes lasting several weeks has been reported.4
HHV-6 type B was responsible for illness in most of the prior reports in immunocompetent patients, as well as in our cases, while HHV-6 type A is usually detected in patients with HIV infection. Primary infection with HHV-6 is almost ubiquitous in infancy, and is associated with febrile illness, including exanthem subitum (also known as roseola infantum or sixth disease).1
HHV-6 remains in the host for life after primary infection. While the actual site of latency has yet to be established, monocytes,14
macrophages, and early bone marrow progenitor cells20
are likely sites for persistence.
In this study, we demonstrated the presence of HHV-6 viral particles in CD4-positive T lymphocytes in lymph node biopsies from symptomatic patients. This is the first report to demonstrate massive infection of CD4-positive lymphocytes in symptomatic, apparently immunocompetent adults. CD8-positive cells were not infected, although there are reports that CD8-positive T lymphocytes can also be infected with HHV-6 in vitro
EM features of the viral particles were characteristic for HHV-6. So far, ultrastructural studies of the HHV-6 virus in human biopsy material have been very limited.22,23
The most characteristic feature that distinguishes HHV-6 and HHV-7 from other herpesviruses is a prominent tegument layer in viral particles present in the cytoplasm,24
as shown in the present case. Detailed ultrastructural analysis of an HHV-6-infected human T-lymphoid cell line25
showed that HHV-6, like other herpesviruses, acquires an envelope at the inner nuclear membrane and loses its initial envelope at the outer nuclear membrane (de-envelopment). Naked nucleocapsids are then released into the cytoplasm and acquire a tegument and a new envelope with spike glycoproteins (re-envelopment). Our finding of both unenveloped and enveloped tegumented nucleocapsids in the cytoplasm is consistent with the reported deenvelopment/re-envelopment maturation pathway of the virus. In addition, we show for the first time in human tissue clustering of mature HHV-6 virions in membrane-bound vacuoles, thought to represent endosomes. Virus-containing vacuoles have been previously reported at the peak of virus production in the cytoplasm of tissue culture cells infected with pseudorabies virus26
and endosomal inclusions of virions were observed in tissue culture cells infected with HHV-625
and pseudorabies virus.26
Endosomes have been considered to facilitate transport of the virus to the plasma membrane26
or to serve as sites of viral degradation.25
While the function of the clustered virions in the membrane-bound vacuoles is unknown, their abundance in present cases supports the diagnosis of an active HHV-6 infection.
An interesting association in Case No 2 is the diagnosis of rheumatoid arthritis 4 weeks prior to patient’s admission and medication with several drugs, including sulfasalazine. Sulfasalazine is one of several drugs associated with drug-induced hypersensitivity syndrome, which has been hypothesized to cause activation of T lymphocytes followed by reactivation and efficient replication of HHV-6.27
Thus, drug hypersensitivity might precipitate HHV-6-associated viral lymphadenitis in individuals with past, rather than primary, exposure to HHV-6.