CD20-negative DLBCL is very rare. The vast majority of reported cases mainly occur in a few variant subtypes of DLBCL, including PBL, PEL, and ALK-positive DLBCL [8–14]; however, data on CD20-negative de novo DLBCL patients are largely limited and limited to HIV-infected or other immunocompromised patients [8–14]. With the aim to improve the understanding of this unusual entity, we retrospectively analyzed HIV-negative patients with CD20-negative DLBCL from a large cohort (n = 1,456) and performed a matched case-control analysis to compare the clinicopathologic characteristics between CD20-negative and -positive DLBCL.
Early studies suggested that CD20 plays an important role in the control of normal B cell activation and progression through the cell cycle [3–5]. However, the exact mechanism by which CD20 functions in B cells and the role of CD20 in DLBCL remains unclear. Bubien et al.  reported that CD20 functions as a Ca2+ channel in B cell membranes. Although CD20 plays an essential role in B lymphocytes, CD20-negative DLBCL cells can still survive after loss of CD20 expression; the reason for this is not clear. Because CD20 is structurally similar to several ion channels , some CD20-independent channels and/or signal transduction pathways essential for the survival of CD20-negative DLBCL might exist. Further research is warranted to explore the possible transduction pathways in CD20-negative DLBCL.
CD20-negative DLBCL most frequently occurs in a few variant subtypes of DLBCL, including PBL and PEL in the HIV-infected population [8–11, 13, 19]. It appears that immunocompromised patients with these DLBCL subtypes are prone to loss of CD20; however, conclusions regarding the relationship between loss of CD20 expression in DLBCL and HIV infections have been inconsistent [19–21]. In a study by Hoffmann et al. , only 2% of HIV-positive DLBCL patients were negative for CD20. In contrast, Xicoy et al.  reported that 26% of HIV-positive DLBCL patients were negative for CD20, and there were no CD20-negative DLBCL cases among HIV-negative patients in their study. For PBL, Castillo et al.  reported that the rate of CD20-negative expression was higher in HIV-negative patients than HIV-positive patients (100% vs. 83%). The relative lower incidence of HIV infection in lymphoma patients in China compared to Western countries may be as a possible explanation for all 21 cases with CD20-negative DLBCL being HIV-negative in this series. Further studies are warranted to elucidate the precise relationship between loss of CD20 and HIV infection in DLBCL patients.
Pathologically, our results showed that CD20-negative DLBCL is more closely associated with aggressive pathologic parameters than CD20-positive DLBCL, with a higher proliferation index and a higher proportion of non-GCB type. In the previous two studies, 18% of cases with a Ki-67 ≥ 80% and 54% of cases with a Ki-67 ≥ 70% were reported in CD20-positive DLBCL [22, 23]. In agreement with previous results, 27.7% of CD20-positive DLBCL patients in the present study had a high expression of Ki-67; however, in the current study, 70.8% of CD20-negative DLBCL patients had a high proliferation index (Ki-67 ≥ 80%). Furthermore, 77.3% of CD20-negative DLBCL patients were defined as the non-GCB subtype according to Hans et al. . In comparison, the rate of non-GCB subtypes was only 34.4% and 58.4% in our control group and another Chinese study comprising conventional DLBCL , respectively. Interestingly, our study showed that 36% of CD20-negative DLBCL patients were CD30-positive. In contrast, CD30 expression occurred in 10.6% cases of our control group and 4%-17% of conventional DLBCL cases in other studies [24–26]. This could have potential therapeutic implication with the advent of SGN-35, an anti-CD30 monoclonal antibody drug conjugate with significant clinical efficacy in relapsed/refractory Hodgkin’s lymphoma and systemic anaplastic large-cell lymphoma [27–29]. Apart from these findings, there was no significant difference in the rates of expression of ALK, BCL-2, and EBERs between CD20-negative and -positive DLBCL.
Several studies have demonstrated that when treated with CHOP alone, CD20-positive DLBCL attained a CR rate of > 60% [30–33]. Similar results were observed in our CD20-positive DLBCL patients (a CR rate of 82%) when treated with CHOP or CHOP-like chemotherapy; however, our study showed that less than one-half (42.9%) of the CD20-negative DLBCL cases achieved a CR. In addition, the 3-year OS rate for CD20-negative DLBCL was only 35%, which was much less than CD20-positive DLBCL (74.1%) in our control group, and even the 5-year OS rate of conventional DLBCL (52%) before the rituximab era . The reasons for the poor response and outcome to conventional chemotherapy in the current CD20-negative DLBCL series remain unclear. It is noteworthy that although the CD20-negative DLBCL included 5 cases with ALK-positive DLBCL, 2 cases with PBL, and 3 cases with DLBCL with plasmacytic differentiation, which are known for poor response and survival rates [8–14], only 2 cases with ALK-positive DLBCL died and 6 of the above-mentioned 10 DLBCL cases were alive at the time of last follow-up (the remaining 2 patients had no clinical data). We can thus assume that the poor response and clinical outcome of CD20-negative DLBCL is not entirely due to the inclusion of such specified subtypes. However, a high proportion of non-GCB types, a high proliferation index, and frequent extranodal involvement in CD20-negative DLBCL might be an explanation because these factors are associated with biological aggressiveness in conventional DLBCL [17, 22, 23]. Rituximab, in combination with chemotherapy, is the standard regimen for de novo CD20-positive DLBCL. However, rituximab may have little value in the initial therapy of de novo CD20-negative DLBCL patients. A lack of active targeted therapy for CD20-negative DLBCL might also contribute to the poor prognosis. Recent studies have indicated that a higher level of CD20 expression correlates with improved OS in B cell lymphoma patients treated with rituximab [35, 36]. Furthermore, the study by Johnson et al.  demonstrated that DLBCL with reduced CD20 expression had a markedly inferior survival when treated with CHOP or rituximab-CHOP. Further investigations are warranted to evaluate the CD20-independent survival pathway and to develop new optimal therapy in CD20-negative DLBCL.
The IPI score is a useful tool in predicting outcome in typical DLBCL treated with conventional chemotherapy . However, univariate analysis in our study failed to demonstrate an association between survival and IPI. In addition, except for a PS ≥ 2, extranodal involvement ≥ 2, and a SD/PD response to initial therapy, the other clinical characteristics (increased age, advanced stage, elevated LDH level, and bulky tumor) was not associated with inferior OS. Moreover, a recent report showed that an immunohistochemical biomarker, such as BCL-2, can predict OS in conventional DLBCL treated with CHOP . Another study by Miller et al.  demonstrated that DLBCL patients with a high Ki-67 (≥ 80%) had a significantly worse outcome compared to those with a low Ki-67 (<80%). Similarly, we showed that a high Ki-67 (but not positive BCL-2) was associated with inferior survival in patients with CD20-positive DLBCL (data not shown). However, Ki-67 and BCL-2 did not have prognostic value in CD20-negative DLBCL. One possible reason for this negative result is that the biological behavior of CD20-negative DLBCL may be different from that of CD20-positive DLBCL.